I have a lower I consider pre-ban, but a buddy considers post-ban.  His argument is that because it has the stops on the safty selector switch it is postban, but I think it is pre-ban because it has no fill in the back inside of the lower.  It has no holes for sears or anything, but it doesn't have the block in the rear where it would be.  Anyone wanna settle this?
Chew

Check the serial number.  What brand is it?

First off, you need to misspell Pre-Ban.
99% of the people who post here call it a Per-Ban, calling a Pre-Ban a Per-Ban is one sure way to get an answer to your question.
Second off, with 4 posts to your AR15 forum name you should be asking if your AR15 is a machine gun. This will also get you plenty of responses.

Now, to answer your question:

First off you should identify the manufacturer of your Per-Ban lower (see, I misspelled it), is it an Armalite, Bushmaster or a Colt (more commonly known as A,B,C) or is it one of "the other" ones.

Second, you should post the entire serial number of your AR15 lower except for the last two digits. Something like 1234XX.
That way someone else can do the search for you. I like to send people to this link:
[url]http://www.ar15.com/articles/article.html?article=21[/url]
Some people will send you to other links, they all say the same thing. Oh yea, this link will take you to the AR15 legal section on serial numbers.

Stops on the selector, no blocks, machined out areas, these are NOT good indicators of Per- / Post-ban weapons.

I really hope that your mystery lower is in fact a legal Per-Ban AR15 lower.

Good shooting!

You may also call it a "pre-band" if you so choose.

According to ATF regs. It had to be a built up rifle before the ban went into affect (9/13/94)
If this reciever was never built up, it's a postal-ban!