Counterfeit detection pens can only test the paper (which is actually not paper, but more like linen). If someone bleaches the ink off a $1 and reprints it as a $100, the pen won't help.
Cashiers ought to be trained to look at the watermark, the location and writing on the security strip and the color-shifting ink on high-denomination bills. It takes literally 2 seconds. But since businesses don't seem to bother training them, businesses will continue to refuse to accept big bills.
Back in the day I worked in a convenience store and they had no rule against taking big bills, but we were trained on how to verify them and if we ever took a fake we were told it would come out of our pay (a bluff, but effective). People didn't use them often but we didn't refuse to take them.
I help a friend who sells costly items at festivals and to my knowledge no seller has ever gotten stuck with a fake bill. Every seller inspects the high value notes and does so in a showy way - I don't think anyone would even try.