Yes, it IS easy to tell. since the only way you can tell if someone has posted or not is on the members list, which also shows their date of registration, its easy to do some math
Anyways, though theres a mass of them, non-posting users take up such a petty amount of space its not really worth it. Anyways, its only a problem if you just stare at the users list, going through for some pointless reason...