A beard on a muslim. If a muslim has no beard chances are quite high that he isn't a jihadi (unless he is preparing to blow himself us because in that case he might have shaved the telltale jihadi beard off in order to blend in better in the crowd) but a muslim with a beard is pretty much always a jihadi/islamist.
Note that it's not the beard in itself but rather the combination beard and muslim that makes the beard a jihadi beard.
It's like the swastika. If you see a SE Asian buddist with a swastika you know he probably doesn't have it because he is a nazi but rather because it's a symbol for the sun in their culture, but if you see a white guy with a swastika fore head tattoo then he is guaranteed a nazi.