Recent indications that people on the street imagine I am Czech include a young woman who requested (and received) directions to the Městská knihovna and an elderly man who was having difficulty getting his balance after getting off the tram and asked if he could steady himself on my arm (I assented). I was also recently asked if I was already in line to get a shopping basket at Tesco, and got to hear complaints about how slowly the line moved and the ridiculousness of having to do this. I entirely agreed. At most Czech supermarkets, one can pay to use a shopping cart, but this strikes many of us as stupid, especially if we only wish to buy what we can carry. The grocery section at Tesco does have wire baskets for people like us, but they are in great demand and you have to stand and wait for an earlier shopper to hand you one. The supermarket at Nový Smíchov at least has employees on roller blades (!) who bring stacks of baskets to a basket area. The shopper might get run over, but will at least get a basket.
Anyhow, I suppose that standing in line at Tesco to get a wire basket does mark one as a person in the know. Maybe that's another major element in appearing Czech (or anything else): knowing how things are done. I wouldn't say this is always easy, but sometimes (as with baskets) it's just a matter of acting like everyone else. Keep your voice down, stand in line, blend into the scenery.