a couple years after getting in to the kegging game, i feel like it's a sense you'll develop over time. tricky little buggers.
-if you don't have a spray bottle for starsan, pick one up. spray the keg and look for bubbles. this will point out the leak if there is one. your leak is probably on the lid as you mentioned. it almost definitely isn't on the beer line, or you'd have a mess on your hands. it could be on the gas line. make sure every point that connects is tightened with a wrench and/or has a hose clamp really tightly applied.
-your co2 tank might be low or empty. this can happen pretty quickly with a slow leak. my first few months of kegging i was going through co2 way faster than i should have been... then i eventually found all my leaks and things are much better.
-if you can't get the leak to seal back up, you'll need some replacement o-rings and possibly some keg lube. i like to keep all this in stock because a leak is never conveniently timed.
good luck and let us know how it goes.