A trained eye and plain old experience are the best tools. Over the years I've come to be able to spot the differences between the various Colts with relative ease. The easiest way is to just compare the pivot pin hole to the rear take down pin hole. Unfortunately, this isn't always possible without at least partially disassembling the gun in question and some sellers are reluctant to do this if they deem you a "Looky-Lou" or a tire kicker.
With that in mind, the models you are going to want to keep an eye out for are the Sporters. The model numbers will include:
R6601 Sporter Match HBAR (A2, 20" HBAR)
R6530 Sporter Lightweight (A2, 16" light barrel)
R6551 Sporter Target Model (A2, 20"` gov't profile barrel)
R6700 Sporter Competition HBAR (Flat top w/ A2 carry handle, 20" HBAR)
R6700CH (Range selected R6700 w/ scope and accessories)
R6830 Sporter Lightweight 7.62x39 (A2, 16" 7.62x39 barrel)
There is also the R6430 Sporter Lightweight 9mm if you want a 9mm AR or feel like dealing with removing the magazine well block and don't mind the resulting holes from the pins. Personally, I would just leave these as 9mm's as they typically cost more in the first and would be best left as is for the people who do want a 9mm AR.
Note that some of the earliest Sporters still used slab side lowers and large pivot pins. While I own one of them, I kind of consider these to be the least desireable of the pre-ban Colts as they have all of the worst features; Slab side lower, large pivot pin, large trigger pins, and a sear block pinned in with 3 huge pins instead of the single pin as was used on the later blue label guns.
I've learned to never say never when it comes to Colt, but if the gun has a slabside lower it most likely has the larger pivot pin. If it has a pivot pin with screw heads on both sides, it most likely has the larger pivot pin. If it has a mag fence, it most likely has the smaller pivot BUT
it is known there are some lowers with mag fences that still have the large pivot pin so you need to ask and hopefully the seller will be willing to take it apart and show you.
In my experience, outside of online sales and auctions the R6830s are often under-/unappreciated and can be found for pretty good deals. Last year I bought an excellent condition R6830 from a local gunshop for $900 and an R6830 complete lower receiver assembly for $699 from a dealer in California who had advertised it on Tom Bowers board. I have since turned it into an M4 type with flat top uppers in both .223 and 7.62x39. Both of them have A2 lowers with the mag fence and small pivot pins but they were not drilled for the detent. This is no big deal as I am currently having the hole drilled and parts installed by another member for $30. And like myself, you might just discover that an AR15 in 7.62x39 is pretty damn fun in it's own right.
I did notice a Sporter Lightweight for what seems like a pretty good price on another board. Check your e-mail.
Just for reference, here's what a mag fence factory drilled for the detent pin looks like. If you come across one of these it will have the small pivot pin and you're good to go. This is my post-ban MT6731.