Quoted from barakandl:
I worked in retail in a previous life and learned a counterfeit detecting pen is not a good way to check money.
This.
Quoted from herbertbsharp:
People with counterfeit bills tend to spend them in busy places on purchases that will give a lot of change with real bills, like busy bars. The cashier will have less time to realize they are fake.
And this.
I worked in cash control years ago at an amusement park. Every bill that was spent at the park had to go through our hands. We'd get fake bills pretty often, from really awful injet printed ones, to pretty sophisticated bleached $1s and $5s that were reprinted to be larger denominations. In the latter case, the bills pass the counterfeit pen (which is really just iodine. Paper money is made from cotton, and won't turn the iodine black on contact). I've even heard of people treating fake bills with vitamin c to keep the pens from turning black on the bills. So we rarely trusted them. In the cash room, we had bill counters that had magnetic sensors. The inks on bills are very slightly magnetic, and that was pretty much the determining factor of whether the bill was accepted, but it almost never got to that point. It was always pretty easy to spot before it got to that point.
Best way to look for fakes is to either really know your money, and find the small red and blue interwoven threads in the bill, use a blacklight to see the band, or check for a proper watermark on the side of the bill. 9 times out of 10 though, it's really easy to spot a fake just by the feel, and the new blue $100 bills are really hard to fake. I haven't seen a good forgery of one.
At the end of the day though, when making large transactions with people for pinball machines, I absolutely do not worry about counterfeits. Too many chances for you to spot it when you're counting it out on the glass.