I'm trying to clean my old MacBook Air for sell. How do I know it is cleaned?

See the instructions in What to do before you sell, give away, or trade in your Mac - Apple Support

When that has been completed it will be as described below from that procedure:

After macOS installation is complete, the Mac restarts to a setup assistant that asks you to choose a country or region. To leave the Mac in an out-of-box state, don't continue setup. Instead, press Command-Q to shut down the Mac. When the new owner turns on the Mac, the setup assistant guides them through the setup process.