Economic factors play a great factor too. Like cost of living, etc.
Yes, and experience counts for a lot. I know some freelance designers here who charge a very high hourly rate, and others who are employed at a firm and get paid slave wages (a tenth of what the freelancers can get per hour). So it's more complex than just what country you are in.
Great point! Maybe the best measure would be to just look at his/her work. Maybe doing a test before hiring or something like that? You just can't be sure right now. Some charge really high even though they don't have that much experience yet.