Re: How can you tell about a receiver?
Well first off I will assume we are talking about Colts only.
And when it comes to preban/post ban it gets kinda tricky.
In GENERAL terms prior to about 1989 they were actually marked AR-15 and after that they were marked Colt Sporter. The Colt Sporters generally had no bayo lugs but did have flash hiders, pistol grips and collapsible stocks on carbine models. Then came the 1994 ban (post ban) and these had NO flash hiders or collapsible stocks. These were also marked Colt Sporter.
Following the sunset of the 1994 ban the "preban" features were allowed again.
There is some more to it than the above, but that will get you started.