[[XST]O_x]

Not really sure. You can check OnPlayerKeyStateChange, but you need to make sure player isn't falling/in car/in water, and is equipped with a melee.
But I think a better way is using GetPlayerAnimationIndex and GetAnimationName.
Here is a list of animations:
https://sampwiki.blast.hk/wiki/Animations
You have animations for Knife and Baseball(baseball bat), pretty sure there is other melees as well.