Yes, I think people could tell, but I've not tried it. When my hair's longer (2" or so) I have enough density for a concealer to help unnoticed. If I shaved down or clipped very close, I think it'd look like I painted my scalp. Someone on another site posted that it worked for them but IMO you'd have to have some pretty good density for it not to be obvious.
Longer hair when using any concealer is better..............no question in my book.