You can look out for his first attack animation.
It's true that animations can hide/cancel eachother, but you should be able to see their first attack.
1. If the monster is aggressive and attacks first, there you have it.
2. If the monster is passive and the player attacks first, in order for its first animation to be cancelled, you would have to attack twice before his first attack (unlikely).
In rare cases where you attack an aggressive monster (who is about to hit you) first, you should still be able to see an attack animation eventually.
EDIT: If you do get hitsplat API added, you can use the timestamp of the first hitsplat on you for melee enemies.