how do colleges know you're not capping on your app?

Genuine question. I guess if you say you're some Olympiad qualifier, there's databases out there or a list that can confirm your aptitude. But you don't get any sort of "certification" for things like student body, or running a club. Or saying you volunteered somewhere, or did babysitting 8 hours a week. As long as you made the hours real (like don't put practicing violin 40 hours a day lol), fabrication doesn't sound hard. And with the large amounts of apps colleges receive, do they really call every school/counselor to confirm that information?