This question already has an answer here:
can i know which user has give me up vote or down vote ? because in general case some developer give a down vote with out reason.
You can never find that out, unless the user tells you themselves. Votes on Stack Overflow are totally anonymous. Even moderators can't see who has voted for you. They can, however, see the voting patterns.
So the conclusion is: It is impossible to see who has voted on your post ( question/answer ).
An update from Jeff's answer developers of SE can see this part.