Here's a scenario that will have many repairers nodding in recognition.
A client comes into the workshop with an instrument - and for the sake of argument let's say it's a flute. It's handed over and duly inspected, and straightaway it can be seen to have a number of problems. Further inspection reveals a few other less-visible problems - and a play-test gets you down to around low G with moderate finger pressure. Pressing the keys down rather harder takes it to low D, but low C is a dead loss.
It's in a proper state - and after you've done the old sucking in of the breath and the shaking of the head, you tell the client it needs a jolly good service. At which point you're told that little Johnny/Debbie just went through their grade 3 exam on this very flute.

And here's another. You get a call from a client who tells you that their child is about to take a grade exam in a couple of weeks time, and they'd like to have the instrument serviced. Fair enough - so you book it in, perhaps sneaking it in ahead of other work given the short time-scale. But you'll need it for at least a day, because it's not always possible to simply drop everything instantly to switch to another job. But this isn't good enough, because the child 'needs to practise every day' - and so the instrument doesn't get serviced.

Now, in all fairness I can understand how these two scenarios come to pass. The child may well be a very keen student, but not many of them understand the mechanics of the instruments they play - and the younger the student, the less like they are to do so. Much the same can be said of the parents - and indeed, one of the most common phrases I hear from them is "Little Johnny/Debby is doing so well...but I haven't got a musical bone in my body'. This makes it even less likely that they'll be able to spot a problem with the instrument.

In an ideal world the child's teacher would pick up on any difficulties. Perhaps the student's problem with achieving a solid, tuneful low F is down to a problem with the instrument rather than the player? But you'd be surprised at how many clients answer 'No' when I ask them if the teacher has mentioned anything about the state of the instrument. It's a big problem, and one that can be summarised by looking at what we need and what we know.
What we need is for the student's instrument to be in good working order - ideally at all times, but particularly in the run-up to a grade exam.
It can be a stressful time for students, and youngsters in particular can be hard hit by a poor result. Just imagine...your child fails to make the grade by a mere handful of points. Could those few lost points have been due to a faulty instrument?
There may even have been issues that the teacher tried to address prior to the exam, but which were entirely due to mechanical problems with the instrument rather than the student's technique. It would have been a losing battle.
So I think it's fair to say that what we need is pretty obvious - nothing much hugely technical about it.

As for what we know, this is where it gets a bit difficult.
A repairer can look at an instrument and 'suss it out' in a few seconds - and an experienced player will be able to pinpoint problems without perhaps knowing why they're occurring. The latter is true, to a limited extent, even with beginners - they might well notice that something's not quite right...but they might not even think to mention it, and if they do there's no guarantee that they'll get much more than a pat on the head and be told to 'keep at it'. That's not unkind, it's just what parents do when their children complain - and because they don't always understand that instruments can go out of whack simply through fair wear and tear (never mind the knocks and bumps).
And that leaves the parents and the teacher. We've already seen that you can't always rely on the teacher to spot mechanical problems...so that just leaves you, the parent. And what do you know?
Probably about as much as I do when my car makes funny noises, or my washing machine starts to smoke. There's no shame in saying "I know nothing about this' - you only need to know that things break down from time to time, and that some things break down more often than others. Or rather that some things need tweaking more often than others, because it's actually quite rare for an instrument to stop functioning completely. It just gets harder and harder to play.

But how often? This is definitely something we need to know. This is why your car has a 'service interval', and why you go to the dentist for a check-up once or twice a year. You don't need to know anything about cars or teeth, you just need to know when your next service/check-up is due.
For a woodwind instrument the service interval is around a year.
And when was your child's instrument last serviced? If it's more than a year then all you really need to know is that it will need a service. If you bought it within the last two years you might be lucky - a good-quality instrument should be OK, but that's not guaranteed, and if you bought a budget-priced instrument it's even less of a guarantee (in fact it might not have been working properly from the start). That might sound like just a sales pitch for a repair service, but conscientious dealers often carry out pre-sales set-ups because they know that manufacturers don't always get it right...and things get knocked about in transit.

We also know that working towards an exam can be stressful. The constant repetition of scales, arpeggios and set pieces can be mind-numbingly boring - so just imagine how much harder it is when the instrument isn't up to scratch. It's like being given 100 lines...and a scratchy ballpoint pen with which to complete them...while someone pokes you with a sharp stick.

And so here's the simple solution. Assume nothing. As soon as your child announces that they're working towards a grade exam, get their instrument checked over immediately. Typically this will be at least two or three months before the exam date - and that's plenty of time to schedule a service around their practice regime.
If you had the instrument serviced within the last six months you can probably relax - but if it was serviced in the previous six months then it will be worth having a 'pre exam' check-up.
Your regular repairer should be only too happy to provide this service - and as you're a regular client you might not even get charged (and if you do it won't be much...perhaps a tenner or so, if that). It many only involve having them quickly look over the instrument, doing a few leaks tests and give it a quick play test. There might be a couple of adjustments to be made, and these will most likely relate to making the instrument feel better under the fingers rather than anything that makes it play better.

If you've been a naughty parent and you've never had your child's instrument serviced - or have skipped a few service intervals - you might find it a little harder trying to persuade a repairer to do a pre-exam check...but you can always ask. One good tip is to promise to make a firm booking for a service after the exam...but make sure you stick to your promise, or the next time they won't be so accommodating.

And if all else fails and you find yourself two or three weeks before the exam with a child whose instrument is falling apart, the best thing you can do is bite the bullet and have it serviced. Sure, it'll mean the loss of a day or two's practice - but the improvements to the way the instrument responds will far outweigh any work that could have been done in those two days (and besides, there's always the theory to brush up on) - and it will mean the student goes into the exam room knowing that success or failure rest solely with them...and not the instrument.