Find month and year for which bill is not generated for perticular customer

I have two table one is Customer an another is bill Column for customer is custID,name. Column for Bil is custID,month,year, amount. every time bill generate entry is made in bill table for particular customer with custId. for every month there is bill for customer but generation of bill is manual. now at particular month I want to find out that how many bills are not generated for particular customer. i am java developer and new in SQL so i don’t have any idea about it.

Database : SQLite3

Thanx in advance.