Look at their CC resistances on the third tab like another poster said: If it’s a low resistance then that’s self-explanatory but if it’s not there at all, then they’re not resistant to it either. They don’t resist damage reduction, nullification, chance to fumble attack/impaired aim, resistance reduction, reduced offensive/defensive ability, etc.
One exception I can think of is “Reflected Damage Reduction”. That resistance does exist but it is not listed. You’ll have to check their individual buffs on the right side on grimtools. Not like it matters much. Same for health reduction resistance; bosses are absolutely fully immune to that, but the stat exists even if it doesn’t show on the sheet.
Also, some skills like CoF or Solael devotion have -xx% movement speed. Bosses are resistant to that, but I’m not exactly sure if it’s affected by their slow resistance stat or if it’s another hidden stat. Same question could be asked about -XX% attack/casting/total speed; no idea if it works, or how it works.
The game has a ton of hidden mechanics, in fact some of them are still being discovered. It’s frustrating even to people like me who have 2k+ hours on the game and dozens of self-made builds. I guess all you can do is ask if you have any specific concern and someone should be able to answer.