In the forums, hold your cursor over their name.
Now if they do not use the forums then you have to do a pocket query of caches that they have hidden.
Now if they have no hidden caches, I'm not sure how you find their super secret probationary number, I don't think GSAK shows id's of loggers of others cachers.
Also, if in the forum, you can click on their name and it will appear in the end of the URL. I just found this out. Thanks for the other suggestions, though.