Hey Greg,
I'd be happy to answer that for you. I was in the same spot two years ago when looking at an early '28. First off the 101 ran from mid '28 to '31. The first noticeable difference is the length of frame. Non 101's are shorter. The second is the front brake. Prior to the 101 the front brake is absent, as is the brake lever on the handlebars. It looks like a bicycle wheel. The caveat is that the bikes are so old some may have modified/added a front brake or cut the frame(as in the case of the first bike i looked at.) It had a correct frame that was shortened, an added front non original brake, and a police motor that bolted through the frame. There is a stamped number on the case that you can reference to check if it falls in the 101 range. The frame numbers do not necessarily match to a motor number on the early models. Also, early '20s have different shaped gas tanks.
That being said, there are also differences within 101's. The later models had cool indian head horns, a cross across the handlebars, 3 as opposed to 4 knobs on the gas tank and drop center rims.
To make it even more confusing (or clear) the non US models had a front kick stand, larger headlight and a different schebler carb number.
'28s had clincher wheels (flat and square looking)
There are other things, but this should point you in the right direction. All of the early indians are really cool.
Good Luck!
