Those pens are sometimes unreliable and the SS doesn't even advise using one. These pens detect the starch in regular paper and witty counterfitters have been known to spray their fake notes with a chemical so when the pen is used their notes appear to be genuine when in fact they are not. With that said though, if you just want to have fun experimenting with this pen then by all means please do so.
recently, my fiancée told me a story of a $50 bill that was received at her workplace. the cashier accepted it as it passed the pen test. but my fiancée said something seemed "off" about the bill. she held it up to the light, and the watermark was of lincoln. these pens only go so far to detect a fake.
I paid $250 for a nice counterfeit bill counter, that is guaranteed to spot 99% of counterfeits, but I returned it because it was a "bill counter" and not a "cash counter". AKA, it would only tell me how many bills I put thru it, not how much money I had. I could put 5 $20's thru and it would only say 5 bills. These are IMO, the most accurate way to detect counterfeits. They do have more inexpensive ones that go for around $100 that have the same counterfeit detection, they just don't count bills as fast.
