You're NOT supposed to be able to check for enemy Operatives since it totally defeats the purpose of having the disguise in the first place. The only way for people to see through disguises is for them to be an Operative themselves or by being a person who the Operative is disguised as.
Having Friendly Fire on with vote kicking available is probably going to be the best option for keeping disguises viable (in pubs at least, future comp chat "GUIZ, OPERATIVE KILLED ME, LOOK OUT FOR MY NAMETAG"). Not really looking forward to the elitist pricks taking advantage of that feature when you make most tiny of mistake though. =I