>How know which records are (aren't) in another table?
>Using SELECT query?
>And when you need more than one criteria?

First, let me make sure that I understood your question correctly. I assume that you have two almost identical tables and you want to retrieve records from table1 which are missed in table2. If it's really so, you run following query:
Select * from Table1 ;
Where Table1.Primarykey NOT IN (Select PrimaryKey From Table2)