You can't do so with perfect certainty, but you can usually narrow it down to which country they're in.
There are several IP-to-country databases which are downloadable free. You can use one of these to look up the country of the IP address they're connecting from, which usually (but not always) means that's the country the user is in.
A few providers do use IPs which belong to another country sometimes - possibly including AOL.
There are more informative, more accurate databases which you have to pay for, which can claim to correctly identify the location of AOL users too.
Mark
