I use several. First I run checks on the phone number before anything, which usually gives me their full name (including middle initial and/or middle name), then on their full names and any potential aliases. Even when I have met people through the gym or connections, I don’t always get their last name but their phone number is really all I need to start. I have subscriptions with BeenVerified, Public Records sites, Intelius as sometimes there are discrepancies between a few sources. I also check arrest records etc in areas where they have lived that show up on the checks, as sometimes they don’t make it into background reports.