How can a normal collector (if there is such a thing) tell a legitimate raw blank back M101-4 or M101-5 from the numerous reprints that are out there? How do the grading companies tell?

Hi Rick! At the top right of the page, there is a links section titled "Detecting Card Alterations and Reprints" The following article is there:
http://www.network54.com/Index/86178

My Trade/Sale Page

Do the M101-5 have a glossy finish? I picked up 7 common cards about 12 years ago at a local auction for $6 and figured they where fake but would look good on the wall in my sports room. D.

Yes, the fronts are glossy on real ones. One of the best ways to tell if the card is a reprint is to look at the white border inside the frame on the front. If it's very large at all then it's probably a fake. After looking at a few real ones that area is fairly easy to distinguish. As pointed out our "detecting" section has a good article on how to spot the good from bad ones.

Are they known to sneak past SGC?

A black light would help. I keep mentioning this, seems like few try it.

Old paper was paper. "New" paper, paper made shortly after World War II,
contains brighteners, that fluoresce when illuminated with ultra violet
light. If you hit an old M101-4 with a black light, nothing happens. If
you hit a modern reprint with a black light, it glows purple, the uv
light is reflected.

A REALLY affordable way to spot some fakes is to use a small, battery
powered, $10 to $15 black light...

If you do that, and pay attention to the ones that fluoresce and the
ones that don't, you soon be able to differentiate authentic from reprint
by a casual look.

I have never personally seen SGC grade a fake M101-4/5....Some of the fakes are pretty good though..

The reprints are easy to spot (unless someone has made new ones that I havent seen yet).

The gap between the photo and the thin black line around the photo is 3-4X too wide on the reprints (because the photo area was cropped slightly when they designed the reprint).