Hearing birds is great but sometimes they don't talk much and you are limited to thier shcedule. I like to actually get out like Strut&RUT says. The evidence of roost trees most valuable is usally obvious when you find it. Wing feathers! The knock those out getting to roost. If you have an annual roost location like R&S described you should find lots of eveidence with droppings and wing feathers. Once you find trees they roost in get to know what it is they like. Sometimes I'll hear birds on the limb in an area. After walking into that area you come across a tree or group of trees that jumps out at you. They are on the side of a ridge, over water or a creek, and just have the limbs and structure that looks right. That helps in confirming what's going on and where to set up in future I find.