Should I check if data exists already or what ?

I'm using MySQL and I want to enter some data into the database.
How should I check if that primary key is already in use ?

For example say I'm using the product part number as the primary key, and a user wants to enter a new product.
Should I do a SELECT query and check if no results were returned, or what...?

I don't use MySQL, so this stuff relates to other DBMS's (SQL Server/Informix etc)

Do you want the user to be able to enter the primary key? if not, just insert a null - the database will work the number out for you.

Otherwise you would probably be best to do the select - you can't be sure that any error message will make sense to the user;)