In ADoM version 1.1.1, creatures can be examined by hitting l (for look), selecting the creature you want to know more about, then hitting m. This will give you a description of it, as well as properties your current character has observed - initially you will probably know its speed (if you've seen it move). This will fill up as you observe the creature type do more stuff and fight it - expect to know its average HP, DV/PV values and so on. Keep in mind that there is a degree of randomness to creatures' attributes, and that creatures can level up. If the short description (before you hit m) says something like "it looks experienced", it's not just fluff.
There are also items and abilities that allow you to learn more of your opponents, but explaining those would be a spoiler.
Like most roguelikes, ADoM requires a lot of player skill and memorisation (or record-keeping): once you've encountered a creature, remember what went wrong, so next time you're prepared or at least know to use your contingency items. Losing is a (major) part of the game. ADoM is one of the harder and more unforgiving roguelikes. Expect to lose a lot, and enjoy your time in the Drakalor Chain!