Why would they need to?
Admissions officers have heard it all. If you are going for quantity, they won't be impressed; they are well aware of how many hours there are in a day and if you claim to be an officer in 20 clubs, it's obvious that most if not all of them rarely meet and get little accomplished. If you do something remarkable, bordering on unique, it will certainly be mentioned in one or more of your letters of recommendation. If not, that's probably a red flag and a quick call to the school's guidance counselor will either clear the matter up or place you in the reject pile.
Rest assured that any truly stand-out idea you can come up with has already been explored by high-paid "admissions consultants" working for rich families. If you claim to have co-authored peer-reviewed scientific papers, they'd probably expect to see a detailed letter of recommendation from the professor you worked with, singing your praises, or largely assume you got the opportunity through family connections. If you write about all the great things you accomplished over the summer in some Third World backwater, they will wonder why you didn't accomplish the same in your own local community and figure it was pretty much just a vacation.
The bottom line is that it's pretty easy for an experienced eye to read your transcript, your essays and your letters of recommendation and quickly see a discrepancy in the pattern
being shown. It's quite probable that many applicants exaggerate slightly, but if every applicant is doing pretty much the same thing, the exaggeration can be ignored as statistical noise. If you're known to lie or cheat at school, your recommendations could directly state that or indirectly reflect it via an absence of specific praise.
Most selective colleges get so many qualified applicants that they could easily fill their class over again with the students they are forced to reject due to a shortage of available space. If you look qualified but something about your application seems just a bit off
, the next applicant will get the nod in your place: You should remember that it is not the admission officer's job to give you the fairest possible hearing, but rather to create a viable and balanced incoming class in the most time-efficient way possible.
Also be aware that admissions officers have no problem whatsoever picking up the phone and talking to the guidance counselors if they feel the need to do so — and that sometimes it can be for a positive reason. One admissions officer called my son's high school guidance counselor because the college was considering sending my son an early-write admissions offer, but didn't want to do so until they saw the mid-year grade report. The lesson here: it's a good idea to keep your counselor informed of your goals and accomplishments, so that he or she can help make your case, should such a call ever come.