One of the biggest problems with dry suit leaks is that the places where your UG is most soaked may have little to no relation to where the water is coming in. Neck seal leaks, for example, result in wetness around the crotch, if you dive in horizontal trim. So my first goal is to try to narrow down the part of the suit that's under suspicion. The best way we have found to do that is to put on something that will really show water spots (this is the surgical scrub thing Kathy is talking about, although any fabric that will show wetness well will do) and get in a hot tub, swimming pool, or I suppose one could even do this in a bathtub in a pinch. Stay there for only a minute or two and get out, and then VERY carefully remove the suit and inspect your underclothing for wet spots.
Once you have narrowed down the site, finding the leak is much easier. We've found the method of obturating the seals, inflating the suit, and spraying the questionable areas with a dilute detergent solution has been the least messy and most effective way to pinpoint the hole.
If you can't find any evidence of a leak using this method, then one of several things is true: Either you are leaking through the seals, the leak is VERY small and you need to stay in the pool longer, or the leak is one that only opens up when the affected portion of the suit is held in a certain position. If you are soaked from head to toe by the end of three dives, I think you should be able to find it.