From time to time, parents will call me and say that friends of theirs assure them that "NYC private school admissions directors can tell if you use an educational consultant." These are often the same friends who advise not to prep for the ERB.
Unequivocally, nobody can tell. (And your child needs a high score on the ERB for private school admission, so unfortunately you may need to prep.)
Private school consultants are like private eyes, psychiatrists, and lawyers. Our relationships are confidential, and our work is invisible. This kills me because I have had some very famous clients whose names I would love to drop, but unlike hairdressers, designers and real estate developers, I can't.
Nobody I've worked with has ever been outed. Nobody can tell if you work with me.
The myth that private schools can "tell" has been spread in print by a very well-known school's admissions director. Every year, when my clients get into her school, I have to give a little chuckle.