You can tell who left you rep by going to "Edit profile" and scrolling to the bottom of the page. The thread, date, comment (if they left one) and the name of the person who left/gave you rep will be displayed there. Oh and of course, it will also display whether they left you positive, negative or neutral rep.

I do believe, if a person leaves you good rep but they have no rep, or little rep then the rep points awarded to you are very low, compared to someone leaving you good rep who has high rep themselves.