How can you tell a printing error or counterfeit is it something to look for i have a 1950 twenty dollar bill with no in god we trust on the back?

The phrase "in God we trust" wasn't added to US paper money until 1957, so of course it won't be found on a 1950 bill. If you're still concerned, take a closer look at the paper itself. Genuine bills have small red and blue fibers mixed into the paper; something that fakes will lack.