So, you catch a 'thief'...

You could go up to him and say, "That's my bike, what the fuck are you doing?!"
If he gets defensive, then he's nicking it, if he looks confused, it's his.

fantastic..but whats next???

this was what happened to me on hammersmith hospital ... and he just legged it...

funny thing was he was there again the next day and the security got him...