How to prove to someone i'm on my period?

Why would you need to prove this? I cannot think of a sensible situation in which someone would ask if their friend was menstruating and not believe them when they said yes.

Is your friend trying to body-shame you for being a late developer or pressure you into a sexual act? If it’s either of these things then believe me when I say they are not your friend.

It's none of your friend's business. Tell them to stop being such a fucking weirdo

Stick your hand down your panties and smear the blood across your friends face. Tends to be pretty effective.

slingshot a tampon at them.

Your friend is a dildo, and you maybe also are a dildo.

Good day.

Your friend is a retard. You don't have to prove anything. XD

I'm with ospry. But some situations in life are so weird you just couldn't make them up.

Roll down the waist of your jeans a little bit so that she can see the top edge of your knickers one day of your period - just enough so she knows what colour the knickers are that day. Then the next day, when you're wearing a fresh pair, allow her to look at the old pair of knickers. There will probably be at least a smudge of blood on them somewhere, even if it's only around the leg elastic.

Even I as a guy know doubting this is unacceptable behaviour. You shouldn't have to prove you're bleeding from your fucking minge to anyone! Just ignore them.

Idk, show them an used pad or the toilet after you've peed.