How do i validate data inserted from asp.net to sql server : I have a web form already connected to SQL Server, i can insert but i dont know how to do the validation to not Insert data if the table already has the same information.

How can this process be done? 


            using (SqlConnection con = new SqlConnection("Data Source=hans-pc;Initial Catalog=CntExp;Integrated Security=True"))
            {
                SqlCommand cmd = new SqlCommand("INSERT INTO PreExp1 (Expe, Usuario) values (@Expe, @Usuario)", con);
                cmd.Parameters.AddWithValue("@Expe", TextBox1.Text);
                cmd.Parameters.AddWithValue("@Usuario", TextBox2.Text);
                //cmd.Parameters.AddWithValue("@Fecha", Datenow());
                con.Open();
                int insertar = cmd.ExecuteNonQuery();
                Response.Write("Inserted" + insertar.ToString());
            }
