As far as I know there isn't a way. All I can say is that units with excellent morale(12) like Classical Hoplites pretty much stay in the fight until the very end. Units with morale 14(Classical Hoplites with 1 exp) are pretty much guarenteed unless they get hammered by every single morale - will end up all dying on the field.

Morale with hoplite and other hard to kill units in generally is kinda whacky if you keep them in guard mode and not moving they take forever to kill so their loss rates become rather low. As far as I know kill rate is probably the most variable modifier.

Other modifiers include:
Scary Unit
Fire
Flanks on 1, 2, or 3 sides
Outnumbered and really badly outnumbered(as far as I can tell there seem to be 2 levels)
Tiredness
% of unit left
Routing friends