So in my campaign, (im not the dm) our DM uses medicine checks mainly to see if we can tell an enemy's current hit points. Could you use Medicine like that or is it illegal?

It's the DMs world they can do whatever. As far as the rules of 5e though, it's not a thing.

You mean they give you the exact HP of a character? That's odd? That info is almost never disclosed. At most people use the old school term "bloodied" which means the creature is at half HP or less.

Info such as a creature's physical state should be obvious and not something needed to roll for. It should simply be part of the narrative of combat. Like "the hags breathing is labored and it looks as if she struggles to stay on her feet"...that means shes ALL messed up!

As for the exact numbers. Nope! What happens behind the DM screen stays behind the DM screen.

Medicine's only RAW use, in 5e, is to stabilize a dying creature with a check of 10 or higher and prevent it from making further death saves.

Elsewise, any use to which a DM puts Medicine proficiency is homebrew.  According to RAW, one cannot use Medicine proficiency even for things such as diagnosing illness or determining cause of death, or any other situation where medical knowledge would potentially be useful.  it is used solely, and ONLY, for stabilizing dying stuff.

Because Wizards is a bad company and should feel bad.  -_-

Quote from Yurei1453 >>

Medicine's only RAW use, in 5e, is to stabilize a dying creature with a check of 10 or higher and prevent it from making further death saves.

Elsewise, any use to which a DM puts Medicine proficiency is homebrew.  According to RAW, one cannot use Medicine proficiency even for things such as diagnosing illness or determining cause of death, or any other situation where medical knowledge would potentially be useful.  it is used solely, and ONLY, for stabilizing dying stuff.

Because Wizards is a bad company and should feel bad.  -_-

Diagnosing illness is explicitly called out in the PHB as something that calls for Medicine proficiency.

Quote from Yurei1453 >>

Medicine's only RAW use, in 5e, is to stabilize a dying creature with a check of 10 or higher and prevent it from making further death saves.

Elsewise, any use to which a DM puts Medicine proficiency is homebrew.  According to RAW, one cannot use Medicine proficiency even for things such as diagnosing illness or determining cause of death, or any other situation where medical knowledge would potentially be useful.  it is used solely, and ONLY, for stabilizing dying stuff.

Because Wizards is a bad company and should feel bad.  -_-

 It does say in the PHB and DM's Guide that DMs are the final arbiters of the rules and can apply them however they see fit according to interpretation. It's not necessarily "wrong" or even "homebrew" for a DM to say "yeah I'll let you use your medicine skill to examine the blood spatter at this crime scene to try and ascertain what happened", that's just a DM doing their job and applying the skills based on the circumstances in a way that makes plausible sense and allows them and the players to tell a good story. 

Hey, time for another iteration of my favorite rant!

Worry less about what a particular roll will do, and more about what a character can do. The way the game is intended to flow is for the players to narrate what their characters are trying to do. Then, the DM decides what - if any - roll is needed to figure out the result of the action, and the DC, and if appropriate the player rolls to find out.

You should not need to roll for something your character could obviously do; and you should not be allowed to roll for something your character can't do at all.

Which skill to apply to which roll is always up to the DM, so if they say that it's a Medicine check to assess the health of someone you're fighting then sure. Seems reasonable to me.

Ok thanks all of you!

I think the general assumption, for roleplaying purposes, is that any numerical score is just an arbitration meant to represent some physical aspect of your character. I know it's common to say something like, "I'm down to 12 HP can I get a heal?", but in-character your character doesn't know what a Hit Point is, doesn't know their initiative, etc. But that's just if you're really into the role playing aspect. Maybe in your setting HP is common knowledge and its something that can be discerned numerically... something more comparable to a heart rate or something in real life.