SQL: Best practice for inserting a record if it doesn't exist

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I have an asp.net Gridview that handles insert operations into a SQL database. Records are only permitted to be inserted if they meet a uniqueness criteria, and this constraint is being enforced using unique indexes in SQL server. If the user attempts to insert a record that already exists, an error message is displayed.I'm wondering what the best practice is for implementing this.[*]Check if the record exists SQL side, using IF EXISTS, and locking hints (updlock, holdlock, etc). Return an error code to ASP.net depending on whether the record was inserted[*]Perform the INSERT operation inside a SQL server try/catch block, relying on the unique index to prevent the insert from occurring if the record exists. Return an error code depending on whether an exception was thrown.[*]Perform the INSERT operation SQL side, but without SQL try/catch. Handle the PK violation exception inside ASP.net instead.Normally I'd consider using exceptions to handle valid operations to be bad practice - i.e. software should not throw exceptions unless something is broken. However if the unique index on the table in SQL is going to implement the desired constraint, why bother performing a manual check for existence of the record?