A shapeshifter has kidnapped Obi Wan, taken his place and fooled everyone. Anakin however, figures out the truth and rescues Obi Wan. When asked how he figured out the imposter, Anakin, too embarassed to tell the truth, says that he was missing a prominent scar. The real reason is that the shapeshifter didn't realise Obi Wan was circumcised. Anakin found out when he accidently walked in on the shapshifter in the shower.
TBH that’s actually rather high on the listof ways they would be found out.
Because overall I think Obi Wan is aprivate person. He prefers to be wearing al his clothes in public. Like I don’teven think he would be the sort to strip down to just pants if it’sparticularly hot (like Anakin probably would). So not many people have seen himnaked and there certainly isn’t any public photos of him naked (Satine may haveone or two in her possession IDK (also Quinlan – different reasons. Maybe.)).
However he and Anakin have been basicallyliving on top of each other for a very long time. Like firstly living in thesame apartment which always has a risk of walking into a room with the otherperson without clothes in. Secondly missions on places where the only bathingoption is rivers or communal baths, etc. Obi Wan may be a private person but Idon’t think he’s that uncomfortable with nudity to demand they batheseparately.
Basically what I’m saying is that Iconsider it canon that Anakin knows what Obi Wan’s penis looks like (and vicethe versa).
So yeah VERY LIKEY way the shapeshifterwould be found out.
Doesn’t make Anakin any more comfortable inthe fact that THAT’S how he figured out it wasn’t really Obi Wan. So prominentscar it is (something else Anakin would be more likely to recognise thanothers)
Which leads to the mildly horrifyingconversation where Obi Wan, in passing, asks which scar the shapeshifterforgot. Anakin trying to say one that nobody else saw the shapeshifter didremember.