I've seen a few rifles in local gun shops that I am doubtful of their pre-ban status.  I have previously read that if a Colt receiver has the word "Sporter" engraved it is a post-ban receiver.  Is this true?

The only way to be sure is to check the serial number.  

There are pre-ban "Sporters" too.I've never SEEN a POST-ban "Sporter"

If it's got Colt AR-15 Model SP1 or Colt AR-15 A2 Sporter II on it, it's definitely preban.

There are a number of colt models with "Sporter" in description - sporter target probably being the most common, which are mostly preban, buy may also have been produced after the ban.  If it doesn't say "Colt AR-15" on it, it had to have been a complete pre-ban config rifle before the ban in order to have been grandfathered.

There are also post-ban LEO "Colt AR-15 A3 Tactical Carbine" rifles which are all or almost all post-ban.

Thanks for the replies!