If you want to see if a record already exists, simply query the table with the params that would you have if you were to insert the record.
For example, if you have a username table (UID, FName, LName) and you want to see if a user exists (SmithJ, John, Smith)
Select * From Usernames Where UID = 'SmithJ' And FName = 'John' And LName = 'Smith';
Or if you want to just see if the username exists it's simply:
Select * From Usernames Where UID = 'SmithJ'
In both cases, if you get a record back then it already exists, and if it doesn't exist then the record count returned would be 0 (zero).