Possibly by your IP address. Some have a geographic part that you can look at and recognise if you know here it is (for example mine has "...cosh.broadband.... " in it. Cosham is a town near where I live. I imagine you can get a look-up table.
They might be able to match up information on you from cookies, especially if you allow sites to set them without your approval.
I don't know another way, apart from Spyware.