Vocal birds are always the best sign, but if you're hunting an unfamiliar area, there are a few rules of thumb...

First, look for physical sign. Scout the area as hard as you can during the middle of the day. Look for high traffic areas, and then look for evidence of the roosting area.

Second, try to determine what types of trees the turkeys in your area prefer for roosts, and then go directly to the base of those trees and look for feathers and/or droppings. A good roost area will be littered with sign.

Third, a good roost area typically has at least 2 of the following characteristics:

1) a nearby water source
2) a nearby food source
3) a nearby dusting area
4) the trees are situated either on a bluff or 1/4 down the leeward side (i.e. they will often fly up from the summit, and then pitch down onto the summit in the morning)
5) a nearby green area (Notice I did not say field. Green strut zones are sometimes only 10 yd square).
6) a nearby briar patch or blowdown (i.e. a "thick" area that can accomodate nesting hens...)

Although it often depends on the turkey population and the hunter density in your area, good roost trees are used by several generations of turkeys. Actually, good roost trees are seldom abandoned (if ever) unless something (typically man-made) encroaches on the turkey's perceived sense of safety. Roost areas will often change due to season (i.e. summer vs. winter ranges), but stay quite consistent in most areas of the US...

Hope this helps...

S&R