Epic Level Monsters?

[Renaissance Man]

When applying character classes to monsters from the core rulebooks, at what point are they considered "epic level", and therefore eligible for epic feats, etc.? Is this a measure of their character class levels only? A combination of the monster's ECL and class levels? Some other arcane formula? I've scoured the ELH, but couldn't find a definitive answer. Perhaps I missed something...

 

[Crothian]

Class Level + ecl

 

[Renaissance Man]

Rather what I was thinking as well. You'd think they'd have taken the time to list ECL's for some of the core monsters, then. Fortunately for me, I have that issue of Dragon where they take a stab at ranking them.