I used to work in a cash & Carry in France and you can imagine the amount of snides i used to get handed
All money i took, i put through the counter as it was 99% good at spotting them
The other foolproof way i used to check was the rub my thumb across the top to feel for the rasied print. Once you get to know the feel of the print you will know a snide without looking at them. I found by doing it this way, you could check the note without making it obvious and when 99% of my Customers were do-as-ya-likeys, you would not want to upset them.
They even used to try the trick of handing me a banded grand with one note missing and one folded over to make it look like there was the right amount
Oldest trick in the book, just gotta flick the bundle like a pack of cards and the folded one used to fall out.
still it goes both ways, i was once handed a banded grand in 20's and there was 51. I said spot on mate