The quickest test is to see where the title appears against the background of the field of daisys.  If you have an authentic program, the "o" in the word "of" appears atop a daisy bud (sort of).  The other location I look for is the "c" in music being between the inverted "V" formed by two stems.

Is My Woodstock Program Authentic?
The programs didn't arrive at the festival until Sunday, and by then there weren't any vendors left to sell them. They ended up driving along the festival roads and throwing bundles of them from the truck. After asking no one in particular if if was okay to take one, I picked one up. I still can't believe how I managed to get it home, and in pretty good shape too.
The "fake" above shows how the title is shifted to the left and is out of the proper alignment.  The colors are way too yellow and the green isn't nearly as lush as the original's.
Look at the yellow paper that is watermarked. Hold the sheet up to the light and you'll see long lines running from the top of page to the bottom. Left to right are shorter lines, interrupted by long ones.

The most blatent copies use a yellow paper that has no watermarks of any kind. I've seen some reproductions that were ambitious enough to get watermarked paper, but the orientation is exactly the opposite of what it's supposed to be.  

Along with the embossed lines (which printers call "laid" paper), the genuine program's paper includes a trade watermark, which I have been unable to decipher (Does anyone know the name of the paper company?).  

Most of the fake programs were originally labeled as reproductions, packaged with premium vinyl records or other promotions with no intent to deceive. Misrepresentations I've seen on ebay are usually of the "I'm no expert, so I'm not sure if it's real" variety. "Buyer beware."