Old cardboard also feels different. If you are a klutz like I am and have bent a prewar card or two then you know that it feels different when it bends--brittle. As for reprinting using old stock, I don't think anyone could print mechanically using 100 year old card stock and not end up with a mess on their hands from the delicate nature of the materials. They could age new stock artificially after printing but then DRC's point on printing tech detection kicks in. Bottom line, though, is that you have to learn what the issues look like and feel like so you can detect fakes firsthand.
Now, as Kevin's site shows, an artist can definitely do miracles with fakes and rebuilds (ever seen some of the Darby Chocolates rebuilds? They are beautiful), but with the hundreds of hours it takes to make a project a success, I don't think it would be economical for someone to try and make high caliber fakes from scratch other than for the elite cards, which would then be subjected to correspondingly tight scrutiny (we hope) by PSA/SGC.
Which brings us to third-party graders (TPG). It is possible to counterfeit any fabricated item like a TPG holder or flip, the question again is economics. To create a proper fake TPG holder you would have to create an injection mold to make the holders and invest in a sonic sealer to seal them. Injection molds are expensive to create and perfect--lots of capital up front. You would also have to create the flips. It could certainly be done but only as part of a concerted industrial effort, not some dude in a garage on weekends. Then you'd have to create the cards to go into them, which requires an effort of an entirely separate dimension. What is far more likely is what the A-Holes at When It Was A Game were doing. Those jerkwads were opening PSA slabs and inserting fake flips with higher grades, then resealing them and selling the cards. They got caught and pleaded guilty to mail fraud. Here is a link to an article at PSA's site on it:
http://www.psacard.com/articles/arti...id=3741&type=1
Now, does TPG catch all fakes and alterations? Hell no, as they will admit themselves in candid moments (well, at least the folks at SGC will; PSA's overlords I don't think believe in the concept of human errer [sic]). But they will give you a shot at weeding out the vast majority of amateur hacks when buying mail order.