Spot-the-tourist is a great game, in which one can demonstrate their powers of perception, worldliness, learning and to a lesser extent, either confirm or disprove any national steriotypes they may have.
Obviously, it is best played in areas where tourists are common, and where they come from as many different parts of the globe as possible to be spotted. Central London is a pretty good place to start, but smaller tourist areas such as Cambridge or Edinburgh - in fact any Capital city would do. Just note that no points are available for spotting foregners when you are a tourist yourself. Spotting Italians in Milan, for example, although seemingly successful, would just be stupid.
Best played in a group, the trick is not just to spot the tourist, but to identify where they come from. The real challenge is to work this out before hearing them speak, although this is difficult in the case of Amaricans, who can usually be heard a mile off, so don't count.
Points can be earned, however, by identifying Americans or other non-native english speakers trying to 'blend in'. Guessing what city or state they come from would also be quite credable.
They come from Baluchistan
Apart from impressing your frends by identifying the nationality from a distance, obviously more points are available if the visitors come from more unusual places. Don't try to stretch you credability too far though. Telling you frends
Is likely to need some justification.