This is kind of a soap-boxish answer, but I think it is your characters that should be trying to find answers to those kinds of questions. If your characters don’t find the answers, than play accordingly. Also, while the generic creatures do have that quality, the ones you are up against may be slightly different- always a DM’s prerogative. So, find out as much as you can about them IN GAME is my advice. Which is well intentioned, not just soap-boxish- if you go in all tooled up for generic creatures, you could really get burned, so to speak...
Cheers