A good way to check the gap on your mill is to use a feeler gage. They are typically used to check the gaps on spark plugs and can be purchased at automotive stores.
But if you are looking for a rough check using common household items, the following can be used.
crisp dollar bill = .005"
standard credit card = .030"
standard 1" paperclip = .034"
dime = .057"
quarter = .070"
nickel = .078"
The coin thickness are measured at the rims.