Registration for campus tours is usually done on the university's website. When you reach the campus for a scheduled tour, there will be a check-in somewhere to confirm your attendance.
Some applications have space to indicate your interactions with the university like how you got to know about them, information sessions you've attended, and campus tours.
If not, you can write about something you saw/heard/experienced in the campus tour that makes the school a good fit for you in the 'Why This College' supplemental essay.
The purpose of campus tours is for you to determine whether the campus is a good choice for you, not to strengthen your application, which is usually an entailed advantage. So don't worry about them knowing/not knowing about your tour.
Hope this helps.
