I admit.. I'm one of those people who thought the "counterfeit pens" were the answer to fighting counterfeit cash! I'm sure that's because I've seen convienience store clerks use them and then put the cash in the drawer because the pen said it was real. But the pens are really giving a false sense of security. Scam artists are learning if they can wash smaller denominations and reprint them as 100 dollar bills, the pens will still show them as real cash.
And believe me, you do not want to accept a fake bill... there's no agency that will refund your money. So your best bet is to inspect the bill with your eye.
The Secret Service has a good (detailed) guide to help you do this. If you own a business, print out their guide and have it handy - just in case!