Found it!
This link was the one that I was thinking of that defined the criteria for epic classes:
Cleric: If you have 24 or more ranks in Knowledge (religion), and can cast miracle as a divine spell,...
Druid: If you have 24 or more ranks in Knowledge (nature), and can cast summon nature's ally IX as a divine spell...
Sorcerer: Knowledge (arcana) 24 ranks, ability to spontaneously cast 9th level arcane spells... (although it is really "cast without preparation" isn't it? A pedantic quibble)
Wizard: Knowledge (arcana) 24 ranks, ability to prepare and cast 9th-level arcane spells.
Other skill ranks could be added to this. And probably caster level 20 as well. And as for the other classes, well:
Epic Barbarian could be made contingent on BAB +20 and 6 rages/day.
Epic Paladin could require smite evil 5/day.
Epic Bard: ?
Epic Monk: ?
Epic Ranger: 5 favored enemies
Epic Rogue: 10d6 sneak attack damage
In general; an 18th level fairly distinctive ability, or a 20th level ability that can be attained by feats (Extra Rage, Extra Smiting, etc.) A lot of rogue prestige classes give bonus sneak attack damage, so I don't think it would be wrong to require it. And if worse comes to worst, they can take a few more levels of the base class before they take the Epic Class.
I'm not sure about the epic bard; I can imagine all kinds of "bardy" prestige classes that might not grant that ability. And as for the monk, levels 11-20 look like a prestige class already. There is a lot of ways that one can develop the idea of an unarmed martial arts fighter. Having them multi-class and eventually go into Epic Fighter might not be terrible.
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