Thank you for your post!
I'm going to move this thread from the FileMaker Community Feedback Space, which is specifically for input on the Community itself, to the Discussions Space where you should receive more views and potentially more feedback on this topic!
Alot depends on the structure of both tables.
The ideal setup would be if the master parts table and the import table both have part ID primary key.
if this is true then a simple not equal relationship will allow detection of not matching records from either side of the relationship. If this is not the case then we need to know more about the structure of both tables
Assuming that same unique part ID (primary key), you can set up a relationship using the default = operator and then do the following:
put the part ID field from the master parts table on your import table layout.
Enter Find Mode and enter an asterisk into that field.
Select the Omit option and perform the find.
Your found set thus produced will be all records in the import table that do not match to a record in the master parts table.