As a general rule, every encounter will have some sort of "sizing you up" segment, often including parley (with intelligent monsters only, of course). In my last post, I dealt with morale checks, but that is when the battle is already on. What determines if a monster will engage in the first place?
For the average monster, the average human is definitely a prey animal, 10AC soft fleshy deliciousness just waiting for the taking. BUT.... any intelligent monster is also likely to be aware that some humans have magic, weapons technology, and advanced monster-hacking capabilities.
So... the intelligent monster will be very cautious around unknown humans, and size them up before blindly atacking. Dinosaur? Giant croc? Yes, they will blindly attack anything smaller than they. Dragon? No, no way. They are smart enough to size things up very carefully before any committed action.
A dragon, for example, would read the general fear level of any humans it encountered. It would seek to determine if they had any valuable treasure. It would scan them for dangerous weapons and protective armor.
If the dragon could "bully" its way to a treasure payout, or even a sacrificial meal, it would probably prefer to do it that way.
Before it came down to full-on, hand-to-hand combat, it would seek to probe the party's defenses, say, through a distance attack, like a breath weapon or spell. It might toy with the party from above. Dropping rocks or trees, or causing rock slides in hilly areas.
It might lead other animals into an encounter... Dragons, for example, are well-known to enjoy "Giant baiting". Upsetting the lair of some large and dangerous animal, and leading it right into the laps of the PCs, might well be in its bag of tricks.
In fact, any of the above tricks might be used by ANY intelligent monster.
As a general rule, a monster, even an unintelligent one, will not attack a party displaying more HD than itself. Thus, a Wemic (Lion-centaur), with 5+8 HD, would confidently battle up to five average humans. A small group of 4 Wemics would look at any small-to-medium sized group of humans and be like "Yeah, we can take them."
Any higher-HD monster will be very used to bullying and terrorizing humans. If they encounter a human group that does not immediately run, they will be highly suspicious, on-alert, and nervous. Leading to the "sizing you up" moment and the attempt to assertain strengths and probe for weaknesses.
Longer-lived creatures are especially prone to this "cowardly" type behavior, as the MM describes the nature of dragons. They have a long natural life in front of them, they are less likely to want to risk throwing it away. Juveniles would be the most likely to display aggressive hehavior. The adult and older monsters, the least likely.