The most common way in reality is to do it with the naked eye. Often there is a distraction, a light hit or something that is used as a wrong direction for theft. This kind of thing is part of the Sleight of Hand ability, that's what it is for.
If your player does not want his target to be able to remember what happened and infer who is likely to have stolen it, then he can request an additional stealth check to determine if they were seen.
But failing in the Stealth Check would not fail in the robbery, although your player can, by finding his target looking at them, decide not to continue.
(Also, if it's a time since the theft occurred, it might make more sense for the target to perform a "memory" check, although, as far as I know, there are no rules governing such things)