Well...Usually the best indication that you have a leak is water dripping... Then again, maybe it could be condensation. I suppose the way to tell the difference would be to wait until later in the day, when condensation should have evaporated, and check to see if you still have a drip. Has it rained recently? If so, maybe you just have some water trapped in the belly, and it has to drain out.
If you have a consistent drip, then there are a few things you can do to troubleshoot:
Are you attached to shore water? If so, disconnect and wait to see if your drip continues. If it stops, at least this eliminates a leak from the fresh water tank.
Drain your black and grey water tanks, then do not run any additional water into them, and wait to see if you still have a drip. If it stops, then this suggests a leak from the grey or black water system rather than the fresh water system.
Open up all of the cabinetry and look for water dripping/pooling underneath.
Check any exterior exposed plumbing/tanks/dump valves.