With US bills, you can use one of these... costs $5.50: http://www.officedepot.com/a/products/548370/Dri-Mark-US-Cou...

Also, on $100 bills, Ben Franklin has a ghost image that can be seen holding up to the light.

NO! BAD! Do not trust the pens. All they do is check the acidity of the paper which is one of the easiest things to fake.

The best way to test is with a strong neodymium magnet. Magnetic ink is one of the last frontiers that counterfeiters have yet to break.

But better than all of that is to require the transaction to happen in a branch of your local bank, or post office, and be funded by a money order from the bank or post office.

All of those elements are visual. You just need to look at each bill for a second to see if it's fake.

Some people have visual impairments but they really shouldn't be dealing with large volumes of cash.