Most colleges require you to send scores directly from the testing agencies so it would be obvious if you have sent scores from multiple test dates. If the school allows self reporting, they ask for the test date that you achieved the score, so again, it would be obvious if you used scores from more than one test. I think if they accept you and you decide to attend, you are required to follow up with the official scores from the testing agency even at schools that allow self reporting.
This isn’t what you asked, but my understanding is that while most colleges allow superscoring of SAT scores, they don’t always allow it with ACT scores so I would check what the policy is at the schools you are interested in.