How about this.
All speedway fuel tanks have 2 fuel taps but a rider has a tank with a secret compartment just for his Nitro.
Prior to his race he's fills his carb with Methanol and goes to the line therefore burning off his Methanol, at the line he then switches the other tap on containing Nitro to then fill his carb with Nitro ready for the start, once his carb is full he then switches that off and he's back on to Methanol and away he goes.
If and only if a riders fuel gets checked by dipping a shringe into his to fuel reaching only one part of his tank it appears to be only Methanol.
All riders when at start line always check their fuel is on so to see a rider doing this would not look suspicious.
All bikes before a meeting are scruteneered so why not scrutinise their fuel tanks too and while they are there check bore size too