This is really an SQL question. What you would do is:
Set @Name = 'a'
Set @City = 'b'
Set @State = 'c'
Insert Into Customer (Name, City, State)
Select @Name, @City, @State
Where NOT EXISTS
From Customer (NOLOCK)
Where Customer.Name = @Name and Customer.City = @City and Customer.State = @State
Select CAST(@@ROWCOUNT as int)
If the result is "0" then it was not inserted because it already exists. Likewise "1" means it was inserted.