Kevin did a nice job of providing a quality answer here for how to find the leak if you are at home. However I usually realize I have a leak when it deflates under me in the middle of the night. If you're not near home (or a bathtub, or other large pool of water), is there a reliable way to find the leak in the back country?
There are 3 ways I've found of finding a leak in an air mattress:
Soapy Water - It may not be likely, but perhaps you have some camp soap or something similar that you can use to make a soapy mixture that will bubble near the leak.
Submerged - I know you said not near a large body of water, but even if you have just a small stream you may be able to dam up a small pool and submerge parts.
Listening - I've found slightly over-inflating (and sitting on it, if possible, to give some extra pressure) and just listening for the leak works surprisingly well. Particularly if you are far from the noise of civilization, it should be pretty quiet, so this works better than I would think if I hadn't tried it in the past.
With any method, the seams are the most likely location of a leak, so check there first.