you look for it. break up a bud, if you see mold, you got it...or the expensive route of having it lab tested.//static.grscty.com//public/style_emoticons/default/confused_2.gif
The leaves will shrivel and curl and the bud will turn brown and be very mushy. Leaves turn brown too sometimes. Cut it off immediately.
You can usually smell it well before you see it, a faint ammonia like odor. By the time you see mold it's already been present for a period of time. No, mold is usually obvious. You either see it on the bud, smell it, or definitely taste it if you smoke the stuff.
you can usually spot it pretty easily. Usually looks brown and manky and will smell noticeably different