How is AI checked by admissions?

So I was watching a webinar on AI usage during college admissions and one of the college admissions officers said that they can tell and find out if you have used AI during your application process especially for your personal statement. So I was wondering how can they? Since when I was checking my personal statement with AI checkers different checkers said that it had no AI and others said it did! So now I am wondering what do they do?