They don't need to be feeding on naturals on the surface to catch them on dry flies. You won't find them doing that very often.  Occassionally you will see a fish take something though.

The best time is when the water is clear and warm during the fall. 50 to 65 degrees.    I've caught them on dries when it was colder than that, but it's rare.   The best time always seems to be when the most leaves are falling, which is a real pain.  I've had fish interested, the fly hooks up on a leaf, and the fish takes the fly,leaf and all. Usually they turn away and leave you dissappointed though.

Flat sections of water are best, the middle and tail outs of pools, occassionally I've taken a few in the head of pools and pocket water, but not as regularly there.

Flies have mostly all been mentioned already. I don't think it's that important what fly it is.  It's usually an oppurtunistic bite, hey what is that a bug, think I'll try and eat it.

Not that hard to try it,  usually if there is a fish in a pool that's going to take it, it will do so in the first few good presentations, or at least show some interest in you're fly.
 
It is much tougher to get them to take a fly, completely floating, dead drift on the surface, than it is to get them to take a fly skated acrossed and down at an angle, or twitched.  However they will take it on a dead drift on occassion, usually shortly after it lands in the water. I'll make a cast, let if float a few seconds, then add some action.