All you need is a penny. Takes about a minute and tells you whether you're fine or overdue.
Take a penny and push it into a tread groove, Lincoln's head upside down and facing you. Then tell us what you see. Check a few spots across the tire — tread often wears unevenly, and the worst spot is the one that counts.
If one part of the tire is noticeably more worn than another, the tread depth isn't really the problem — something is causing it.
Fitting new tires without sorting the cause just wears the new set out the same way. Bring it by and we'll tell you what the wear pattern is pointing at.