Well, I have now had two quite different experiences with these people.
Number 1
A week ago, I had been in Praha for a few days only and was only just getting the hang of the transport system. I was on the bus and a middle aged man came up behind me and mumbled something. Then flashes his inspector's badge and then again, so I get out my ticket (all validated and everything of course) and he checks it and moves down the front of the bus and gets off at the next stop without having checked ANYONE else's passes/tickets. So I'm thinking, it is that obvious that I am not a "local person"? And the answer must of course be yes. Why? The fashion obviously is not quite right. See post on fashion for the ins, out and whys and my tips on fitting in.
Number 2
This was this week and it was a much smoother experience. I think also the inspector was nicer... He got on the bus and checked everyone's tickets, not just mine, so I felt better about it, I guess it is just an individual thing...