I have been thinking about picking up a new rifle soon. I am interested in getting a used AR if I can find a good deal on one (possibly a pre-ban because I do not live in a free state (i.e. a forever-ban), and only pre-bans are allowed the goodies...SO....what are the best ways to tell if a Rifle has been shot to hell and is generally worn-out? Any specifics to watch out for? I want to make sure I am getting what I am paying for.

finish wear, bare metal, scraches in the magazine port/ upper rec etc

I would look at the bore, and the over all wear on the parts. You have to shoot the piss out of an ar to wear out the expensive parts. The main things would be the bore, and any damage to the upper or lower recievers, cracks etc.

The lower receiver is all you really care about - barrels can be replace pretty cheaply. Other than excessive wear, you'll have to trust the seller on round count and what style of shooting he has done through the upper.

Check the extractor.

wear around the end of the muzzle.

wear and dings and such on the stock, if it is a wood stock.

wear on the blueing....

Things like that...

If you don't know, take someone with you that does.

AB

Cam pin is a pretty good source of evidence, as are the bolt carrier rails. Of course the bore is a good place to check.

I would trust AR's more than I would trust high power/Wildcat rifles from a handloader. And the nice thing about an AR is that it is modular. Bargain down any excessive wear, and you can replace those parts.

Quoted: The lower receiver is all you really care about - barrels can be replace pretty cheaply. Other than excessive wear, you'll have to trust the seller on round count and what style of shooting he has done through the upper.

$200 for a decent barrel is not cheap!

For an AR?

Check the bolt and barrel extension. The bolt should not have any rounding of it's locking lugs, at all. The extractor should not be chipped or worn down. (To compare, buy a new extractor, and compare the edges/surfaces against a really used one...you will see a difference.) The buffer should not be beat to crap, and the buffer spring should be within spec. On an AR, the lower does not take a horrible beating except around the front and rear takedown pins..they should be checked, and their holes, to make sure they haven't been deformed.

The upper is the part that really takes the wear. The lower is the most expensive part, but it's the part that receives the least wear. External finish is not an indicator of function (I have a seriously worn beat-to-crap lower, that runs just fine.)

YMMV.

On an AR there's really only 3 things you need to be concerned about.
1) Have the head space checked. If that's ok it will safely fire rounds. If you don't have your own head space gauges a good gun smith should be able to do it for you.
2) The bolt cam pin. If it doesn't have a groove worn in it you can reasonably expect it to be ok and not break causing the a malfunction. If you don't already know how to remove and strip the bolt to visually inspect it refer to the instructions link from the AR15.com home page.
3) Function check the safety / trigger action. A worn sear / hammer / trigger will cause a malfunction. If you don't already know how to do this refer to the instructions link from the AR15.com home page.

If those things are good you can start having fun. There is no practical way to tell if the throat and barrel are shot out without shooting it with good quality ammo. The rest is all cosmetic. If you want something that looks new and unfired, get a new unfired rifle.

Remove the handguards and take a look at the gas tube.
The color will tell a lot.
Slightly discolored means not a lot of shooting.
Purplish\heat treated look  means lots of rounds fast.