Once a machinegun, always a machinegun. If it was drilled for the auto-sear, you don't want it. It's obvious if it was. The hole would be above the selector switch. The hole is completely through the sides of lower reciever.
If a RDIAS or auto conector (these are classified as the machine guns, not the host gun) was used and M16 internals were installed, it can be returned to an AR15 semi-auto rifle quickly and legaly.
An AR15 won't be a full auto rifle if M16 internals are installed without the addition of an auto sear, RDIAS, or lightning link. Of course the ATF would say different. Just make sure the internals are semi-auto Ar15 internals. The bolt carrier, trigger, hammer, disconector, and selector switch are all different.