There is no surefire way for Google Forms to tell if you cheat. However, there are some indicators that may raise suspicion. For example, if you submit a form and then immediately submit another form with identical answers, that could be viewed as suspicious activity. Additionally, if you consistently submit forms with very high accuracy, that could also be viewed as suspicious.
Of course, there is always the possibility that someone could simply be very good at filling out Google Forms! If you suspect that someone is cheating, you can always reach out to the person directly and ask if they are sure that they are following the rules.