There is in fact such a thing as a stupid question. "How could a college really know if you lied on your application?" is a good example of one.
The problem with that question isn't that the answer should be obvious. It's a stupid question because lying to your colleges is a stupid thing to do. And most students aren't posing the question hypothetically. They're asking because they're considering telling the lie.
Colleges know how to spot inconsistencies in your application. They notice when things you say don't match with what your teachers or counselors say in the letters of recommendation. And colleges won't hesitate to call your counselor to verify information that doesn't seem right. They don't do it to catch you in a lie. They do it to make sure they have accurate information.
So sure, it's possible that you could claim to be a National Merit finalist and the college would never know. You could claim to have played two years of varsity soccer when you only played one, that you did 50 hours of community service you didn't really do, or that you've never been suspended from school when, in fact, you were suspended once as a freshman. A college might never find out.
But the real question is, is it worth the risk?
If you lie on your college application and a college finds out–no matter what the lie is or how they find out–that's it. You're not getting in. And it wouldn't be unheard of for colleges to tell your other colleges what you did. Colleges know that kids who are willing to take that risk are more likely to do things like cheat on a test or plagiarize a paper. So the risk dramatically outweighs any potential reward. And when you sign your college application, you're signing a formal document stating that all of the information is true to the best of your knowledge. So if you get caught, forget it. There will be no apologizing your way out of it.
Nice, confident kids who've worked hard don't ask us this question. So don't let the pressure of college admissions influence you to lie on your college application. Be better than that. It's not worth it. You don't need an admission to Princeton or NYU or UCLA badly enough to lie. Just be honest. Be proud of who you are and what you've done. If you've made mistakes, be mature enough to own up to them.
It's hard not to like and respect people who have the guts to tell the truth.