The non-partisan “I vote, you vote” is offering a way to check to see whether you are registered to vote. At first I wondered whether this site was legitimate, but I entered a few names of known registered voters (including me!) and a few false names and the site was accurate every time. Therefore, if you’re not sure, enter your name. If you aren’t registered, go register! How to register in your state? Check out this site: Election Protection.
The site seems to only work if your name is unique in your state. I can't believe there's no one named Thomas Johnson registered in all of Texas or California (it seems to check the whole state if it can't find someone in the submitted zip code).
So, if you have a common name, keep in mind that "We can't confirm Thomas Johnson is registered to vote" is not the same as "Thomas Johnson is not registered to vote."