The color yellow is disallowed in match, and Super Tails and Knuckles have invincibility stars going around them as well as being yellow. Super Sonic doesn't have the stars, but he has is own sprite and it's pretty obvious.
Also, when someone goes Super, it puts a gigantic CECHO on the screen saying that it happened, so it's exceedingly obvious when someone went super. Also, the tab screen shows the emeralds each player has, and when someone is super it'll show their emeralds as flashing.
It's generally very obvious when someone is super and you need to watch out.