Good question. Are you asking about in r/l or o/l?

If you are referring to r/l, my best suggestion would be to observe how this person interact with others. If they seem to have a way about them that shows confidence and they "draw" respect from others, then I would be more likely to believe they are a true Dominant. In that same respect, if they appear to be argumentive with others when expressive themselves or seem to complain a lot about how others "wrong" them, I wouldn't give them another minute of my time.

As far as o/l goes, I really don't know all that much. However, if the person began a conversation with me insisting that I either call him Sir or Master, or started giving me orders, I would definately be very hesitant to believe they are a true Dominant.

Sorry I can't be of more help. Best of luck to you in your search, and Welcome!!