check if value already exist

Question

i have a table stock with column stock_id,stock_code stock_desceription etc

stock_id is the primary key while i have stock_code is indexes/key i want stock_code to be unique i every row,am able to do so and also am able to check whether stock_code exist before i insert,now my question is how can i be able to check for dublicate before i update,

below are my code that check before i insert

tStock = svcClient.GetLocationStocksByScanCode(MainConn, cbolocation.SelectedValue, TxtScancode.Text).FirstOrDefault

If Not IsNothing(tStock) Then

MessageBox.Show("Item already exist in the location!", "Softbit Tech")

Return

End If

 now what can i do to check before i update