One that I rarely see in the uk is dictating which arm has the arm band, eg red armband on left arm, blue arm band on right arm
Then if you're blue and see a bare right arm then its a fair assumption that they are the opposition
Glowsticks for night or indoor games. (But everyone wants to keep their arms back, or wrap extra tape (of the appropriate colour) to keep it 'more secure' on their arm (otherwise known as wrap tape around the glowstick in a spiral to reduce how much glow is showing)
Glowsticks are handy for medics - I have worn a necklace of 5+ glowsticks - preferably behind my back, and swung them in circles to advertise my location to people seeking respawn
Wearing clothing or something of the wrong colour is a problem if you're not sat at the back.
I have a red team logo, and when we were a team of two at north vs south on the blues the first year at swynnerton we had our patches on our backs (1 inch wide, 2 inches tall) and blue arm bands. We were both shot by friendly fire, not cross fire as they were direct hits, a reasonable mistake possibly - but straight into the back running up he first hill at game on! Not the cleverest at identifying people
At the rental site I first played at they used colluded hoppers, so I would use my gear and site hoppers, then I wanted to use my own (or my qloader) so would bring coloured tape and stick on the appropriate colour
At some indoor / cqb games in addition to the arm band (or glow stick) tape is often put on the end of the barrel. Then if you're at a corner and are quick thinking (or your opposition is hesitant) you can identify friend or foe and be ready for them to come around
This gave me one of my favourite pump eliminations - a barrel poking out at a corner, I hesitantly crept forward from my tiny recess in the wall as near as I dared, one shot right on the tip of his barrel - watching with joy as he twitched on impact, pulled back the barrel to look then left. I retreated like a scared little girl to my recess for the rest behind him to rush me!
A team uniform - as mentioned earlier, combats v jeans, or various camouflage styles for each faction. This works well when everyone co-operates, but is difficult when you have sponsored teams or people have to buy gear for their faction.
Laminated coloured/faction badged cards on your goggles
Faction identity works well with people who comply and look to identify friend or foe. No system works for those who shoot first, engage brain later