No, this isn't a summary of the anatomy of a ukelele. We're talking uke characters from BL games, or certain (most) series with BL potential, or just any old anime/manga that happens to have any percentage of yaoi fangirls.
Note: I could've called this 'How to Spot an Uke', 'You Know You're an Uke When...', 'Become an Uke in Five Easy Steps', or something to that effect, but I like keeping in theme. x3
Another note: Though I'll say 'you must', this list is by no means exhaustive and there are no hard or fast rules to being an uke ('hard' and 'fast' are more a seme's specialties after all xD /shot), so take it all with a grain of salt and (hopefully) have a giggle.