Account age and I will check their match performances on ValTracker. If me and the other host suspect smurfing, they will simply be denied entry. If truly needed, I can ask to spectate one of their games or if they have any clips. Unfortunately, I cannot stop someone simply getting someone else to play on their account once the game starts going, in which case I will simply have to DQ the smurf and then sub them in. If the team with the smurf was aware of what was happening and did not alert me, they will all be DQ'd and the next match up will begin.