Welcome to the ZDoom Forums!

(Please move this to where it belongs moderators, cause I'm not sure.)

I wonder if anyone have created some sort of actor debug tool or something, where maybe you can see the flags over the head of the enemy or so? I use all sorts of ways to try and debug my actors, with changing sprites and whatnot, but sometimes this is a bit too intricate to do, too nested.

Or if such a tool does not exist, how does power users usually go about debugging these matters, just by experience - trusting the system?

You can try console command "info". When you are aiming at an actor (i.e. it is in your crosshair), summon the console end enter "info". It gives you a lot of information about the said actor. Also you can add a binding to your autoexec.cfg, e.g:
Then you can just press "i" when aiming at an actor and then look into the console.

Great, just was I was looking for. Is it possible to bind the same button to two commands? Like Pause and Info, or maybe info+lower console?

Solved it with an alias in the ini:

Name=kanelbulle
Command=clear;info;wait 5;toggleconsole

Alt=kanelbulle

While I haven't used it myself ZChecker might be the tool you're looking for: viewtopic.php?t=78688