During intros how do I know if they're just playing on being mean.

You can definitely tell the difference. "Being mean" is loud, destructive, and can escalate very quickly into a bad situation. Listen for the extremely loud crabbing and total chaos. During intros, just be ready for anything. Be ready to split them up in a second, because that's all it takes for injuries to occur, which means a stressful time ahead for the glider and it's owner.
"Playing" may only have the "pssstt" sound as a way to say, "back off, I'm not sure what your intentions are..."
Just make sure to do the proper procedures before intros. Get each of the gliders used to each others scent, make sure to take the gliders to the vet for check-ups, ect.

You will know! It's just like a cat fight. Be right there with sturdy leather gloves on so you can put your hand in between them without getting torn up. Don't turn your back for a second.

They seriously ball up and will be hard to pull apart. You will know when they do as everyone mentioned above.

mine during intros had two big fights they get along now but i couldn't help but laughing they were rolling all over the tub in like a movie scene manor i got them seperated and no one was hurt

We had taken Sassy out for playtime and she jumped on the other cage. One came out of their pouch and started making noises at her. I wouldn't say it was crabbing although there was a little bit. I know it wasn't barking either. Well when the other glider would do that Sassy would jump straight to me or my boyfriend. Then she'd jump back on their cage like 2 minutes later. This happened oh for about 15 minutes.

Most gliders will "defend" their territory. It sounds like that's what's going on here. I would be careful for the time being. Until you have the 30 day quarantine and vetting out of the way, you can't switch pouches back and forth. After you've done that for awhile, hopefully this will get better, because they'll be more used to each other's scents. I am working on combining to boy colony cages together right now. It's a s-l-o-w process. However, be patient, and it should work out.