The latest in counterfeiting wouldn't make them very useful. Low $ bills are put through an acid wash then reprinted with a higher denom. It's the same type of paper so the pen doesn't pick it up. If a cashier glances at it quickly, they'll see a watermark & the dollar thread. Unless you look closely (where the thread lands in the serial # & confirm the watermark face), it's hard to catch quickly.
Also remember that all bills will be deposited & fakes will be picked up by the bank. If they notice an increase in counterfeits, they'll notify Secret Service & they in turn will contact your store's AP. Takes a while but it does happen.
Not necessarily....my local bank didn't. People were getting $$ at the ATM at the bank, then going next door to the gas station to get gas, where THEY were catching the fakes. Slightly embarrassing for the bank to get caught "not catching" counterfeits :facepalm: