Ideally you should check every Kong note. In reality behavior would probably be similar how bills are verified now; businesses regularly check $100 and $50 note

Ideally you should check every Kong note. In reality behavior would probably be similar how bills are verified now; businesses regularly check $100 and $50 notes under backlights and with special pens but don't worry about lower denominations. A dedicated "Kong scanner" could do this very quickly.