Preset answers (truth or lie) in the settings menu and then ask them questions. Have them place their finger on the fingerprint scanner while answering the questions. The app will display the preset answers.
If you choose to play for fun, just cancel all the settings and the app will respond randomly, alternating between truth and lie.
Disclaimer: This app is just a prank, a fun game and it is not a real lie detector nor a real fingerprint scanner.