It can be really difficult to distinguish heavy condensation from a small leak. The first trick is to figure out if the dampness is fairly evenly spread over the undergarment, or whether it's concentrated in one area. Concentration is more suggestive of a leak, but be aware that where the concentration is may not have much at all to do with where the leak is. If you dive in good trim, leaks from a great variety of places will all result in a wet crotch, because the water seeps down the front of the suit and pools there. (One of the most common culprits for this is a not quite solid neck seal!)
If you strongly suspect a leak, a really great way to find the general area from which it is coming is to put the suit on over a light cotton garment (I love surgical scrubs for this purpose) and get in a hot tub or swimming pool for just one or two minutes. Get out, carefully peel the suit off, and inspect the scrubs for wet spots. If you find one, you have at least isolated the general area of the suit where the leak is. Then you can use any one of a number of techniques for finding the leak. I've had pretty good success with inflating the suit and spraying the area in question with a dilute detergent solution. Where it bubbles is where the hole is. You can also take the suit in a dark room and put a light inside it, or turn the suit inside out and fill it with water (but that technique is really a PITA).
With my first dry suit, I asked the question you are asking for months, and even had my instructors feel my undergarments. Nobody was sure. It turned out there was a seam leak in the crotch that only opened up when the leg was flexed just the right way. It took over a year to find and fix that leak!