Powerful enemies will show a Red Star above their avatar.
Powerful Enemies are the second strongest type of enemy, and will only spawn for those capable of dealing with them. This means you must have high leveled weapons and armor to be able to spawn Powerfuls.
HOWEVER, having lower level weapons does not make you immune to Powerfuls. If someone else spawns Powerfuls and leaves, you may still be able to find a Powerful or two lurking in the shadows. Powerful Enemies DO give more Cash and Experience, but be cautious - a Powerful could easily dispatch you if you aren't prepared. Keep Painkillers on hand.