I was wondering how I would be able to check when the player is eating a corpse. I thought about cheching the anim, but I believe it uses the standard crouch anims.

Any ideas?

a) add something to the start script of the cannibal perk, for example setting a questvar to 1 which triggers whatever action you want
B) poll the GetPCMiscStat "Corpses Eaten" for an increase, if i read the script of the cannibal perk correctly its increased immediately after you start to cannibalize something.

I'd try B) first since it doesnt require any changes to existing records.