You need to find an online checkers site/community where you have a login or account, and the system gives you a rating based on your wins and losses. These are very common for chess and go. (freechess.org is great for example) I don't know of one offhand for checkers though.
Edit:
looks like the american checker federation has a pretty active forum. I'm sure someone there could point you in the right direction.
http://www.usacheckers.com/forum/