You can make a leak light from a battery, couple of wires and a torch bulb, however even cheaper (and better IMO) is to test for leaks with a cigarette paper.
Tear the sticky bit off and discard, then with light pressure on the key, it should be able to grip the paper. You need to try on at least 4 sides of the key. Keys that are held closed with a spring should grip the paper all round with no extra pressure.
Sometimes a leak is caused by a slightly bent keycup, in which case bending it back is all you need, but this is only to be done at your own risk, an experienced tech is the best person to do any bending.
If the leak is because the pad is incorrectly seated in the keycup, then reseating or (often better) renewing the pad is best.
Sometimes a leak is not caused by pads, but by worn or missing corks - a telltale sign is if you can feel some play or the leak is on every side of the pad. Or it could be an adjustment screw. (especially G#)
Before tackling anything like this on your own, I'd advise you just practise dismantling and putting the mechanism back together first, giving everything a good clean and very light oiling of the rods as you do it. That way you learn a bit about how it all works and you may diagnose cork or adjustment screw problems before anything more drastic like taking pads off and renewing them.
Be aware that a bit of adjustment of cork on one key can easily throw the next one out of whack, it can be quite frustrating. Good luck.
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