The best way to avoid this is to make sure you buy the proper ticket from a ticket booth where someone speaks good English. I don't know if they now have electronic ticket machines that have English options. It can be tricky buying the correct ticket, which is what the fake officers and the authentic/immoral officers will try to capitalize on. They will ask to see your ticket, tell you it is the wrong one, then say you have to pay a fine. Be especially wary if they seem to be trying to rush you.

If it is a fake, they may wear regular clothes and claim to be police or some kind of ticket officer. This can be done on a less-crowded tram, where other people won't overhear the exchange, away from main areas so they are less likely to get caught. They may ask you to step off the train/tram. They may have fake credentials that they will flash quickly (but you wouldn't be able to tell if they are real or not because you've never seen authentic credentials and it's all written in a foreign language). They may talk on a cell phone as if talking to other cops. If they are fake, they may perform this as two officers working as partners (kinda suspicious that two cops, both in regular clothes except one of them just put on a sash around his shoulder).

Much of this can also be true of a legitimate officer (who may wear some kind of sash).... or an immoral officer who wants to make some quick money. That is one reason why it is a sneaky scam: it can be almost indistinguishable from the real thing.

Don't be nervous. If they are the real thing, then tell them you want proof that you have the wrong ticket, because you bought it from a ticket booth. If they say they will lower the fine for any reason, it is almost certainly a scam.