If the deer looks up and is facing the road, get ready to slow down. Watch for tensing of its shoulder muscles, whether its head sort of drops down like it's going to spring forward. Inexperienced deer will often stand in that position being indecisive until the last possible minute, and dart out just as you get there to hit them.
If the deer looks up, is facing the road, but doesn't tense, it won't bolt. It might stand there and watch you pass, or calmly turn around and amble back into the trees. (But slow down anyway, just in case it's insane.)
If the deer looks up and is faced away from the road, it won't run out in front of the car even if it does bolt. However, there might be more deer on the other side of the road that will bolt after the first one. Those will be trouble. (Don't honk at them. It just makes things worse.)
If the deer doesn't look up at all but just keeps on blithely grazing right next to the road, it's so inured to cars that you can ignore it. Deer on I-16 (between Savannah and Macon, GA) are like that. Some of them even know how to look both ways before crossing the highway.
Some evenings as I drive through the multitudes of deer on my way home, I wonder: why hasn't anyone domesticated them like we have the cows, pigs, sheep and goats? Why isn't venison sold at every grocery store? How did they get to stay wild and everywhere?