They were numbered, but it was impossible to verify every check at the counter, because it would have taken forever. (Or even impossible. )
I guess they found out the first time they had the same number twice. But even after knowing that there were counterfeits, they basically couldn't do anything in her past life. (Something like that was said in the book if I am not completely mistaken. )

Yk I get how the informant guy knew the cheque was counterfeit (probably thru the people who was making the counterfeit stuff??) but how did the bank know the cheques were counterfeit if they’re indistinguishable??