I found the controllers easy to spot as a foreigner; they look like hardened police officers who will not take your shit if you don't have a ticket.

That's my experience, at least.

When I was young and poor I "saved money" by buying the ticket in the morning, and only stamping it on the way back in the evening. I gambled successfully that if you get caught in the morning then you are the foreigner who does have a ticket (time of purchase is on the ticket) but "was not aware" that you had to stamp it.