First things I look for:
1) The compass, so you can get back to the car.
2) Critters. Whatever it is I am scouting for, I might see them while driving into an area or while stomping throught brush. They will probably be high tailing it out of there, but for scouting purposes that is OK. I know they are using the area and it gives me a reason to slow down and look closely.
3) Tracks in areas clear enough to show a print
4) Trails
5) Beds
6) Evidence of browsing.
How do I decide where to scout?
Get at a map of the area and look for funnels and pinch points like a thin strip of trees crossing a field, boggy or wet areas that they might travel around. If I have a creek, I scout it for crossing points. A chunk of pines in an oak or aspen forest forms an edge, which should be looked at and pines show up well on arial photos.
I often, mistakenly, think the deer are all huddled up in the smack dab middle of a section of land as far from the roads as they can get. So, I start walking into the middle of things taking note of what I see and usually end up following the runways where ever they go.
What tells you that this may be a good area to scout to begin with?
I only know that an area WAS good to have scouted. I don't know how to tell if it WILL be good go scout. Like trying to figure out if the fish are biting: you don't know, if you don't go.
Then where does this take you and why?
Usually, I randomly tromp through an area until I find a nice runway. Then I follow it. If I am lucky, it will become apparent where bedding occurs and where food sources occur. Often it is not nice and tidy like that and it all looks as good for eating and sleeping to me. But the deer will have preferences. Then, armed with that information, go back to the maps and look for ways to approach the feeding and bedding transitions. Pick a couple of approaches for various wind conditions. This is a good way to come up with plans A, B & C, which you should be ready to scrap for plan D when the actual hunting experience dictates.
Seeing actual animals is, for me, the biggest clue, especially if I see them (or hear them running away) in the same area more than once. I may not fully understand why they are there, but just knowing that they are there is enough to come up with a plan to hunt the area.
Good luck,
Doyle