Well. How badly an opponent is hurt and/or stunned is often quite obvious to an experienced fighter. Experienced fighters KNOW how much something hurt by their opponent's body language. So if they ask, I'll vaguely indicate how badly injured they are and certainly how many rounds of stun they currently have active. I tend to do large numbers of combatants - I've actually run a 60+ character battle, so I need the players to help keep track of such things anyway (stun pebbles next to combatants work great - remove one at the end of each round).
But if you like keeping that information from the players (or pretend that they don't know for role-playing purposes), I would work it like this - if the character did the stun, then they shouldn't even have to roll - they should have an excellent feeling for what they've done. If the stun source was from elsewhere, then judge whether or not they saw the blow, magic or whatever caused the injury and decide how much knowledge they have of such things (usually adventurers know all too well what it feels and looks like) and give a perception roll ranging from medium (saw it, know it) to very hard (didn't see it, its a dragon - how many stunned dragons does one get to see?)