I'm pretty good at spotting ticket inspectors in Berlin, plain clothes and always working in pairs. They stand on the platform at each end of a carriage and just as the doors are about to close they step onto the train and once the doors are closed announce who they are. If they catch someone without a ticket they take them off at the next station and process them - show some ID and pay the fine. Repeat offenders get higher fines and if caught four or five times you can expect to spend day or two away from home in a nice little cell.
Also there is no point arguing your case, no valid timestamp ticket then 60 Euro fine. If you could say 'Oh I have a ticket here, I'm a tourist I just got here and I didn't know you had to stamp it' then me and the other thousands of native English speakers would travel for free all year.
Some routes have more inspectors than others, I've seen three sets of inspectors processing people on the platform at Warschauer Straße U-Bahn from one train and yet in nearly five years I've never been checked on a tram.
I usually buy a monthly ticket, the last one was on Feb 15th. I've found out the hard way in the past that if I'd bought it from a U-Bahn ticket machine it would be valid for the month of February. Buying it from an S-Bahn ticket machine as I do (same price) means it is valid until March 14th.
Overall it's great to buy a monthly ticket, stick it in your wallet and then jump on any S-Bahn, U-Bahn, Tram, Bus without going through some barriers.
