Check Before INSERT

I have a pretty standard form that inserts users name, office, and team. It generates a random 10 digit ID for them. How would i got about checking the table to make sure that ID doesn't exist?

Here's my insert code.

        string strConnection = ConfigurationManager.ConnectionStrings["TimeAccountingConnectionString"].ConnectionString;
        SqlConnection myConnection = new SqlConnection(strConnection);

        string usercode = GenPassWithCap(9);

        String insertCmd = "INSERT into users (ID, firstname, lastname, office, team) values (@id, @firstname, @lastname, @office, @team)";
        SqlCommand myCommand = new SqlCommand(insertCmd, myConnection);

        myCommand.Parameters.Add(new SqlParameter("@id", SqlDbType.VarChar, 10));
        myCommand.Parameters["@id"].Value = usercode;

        myCommand.Parameters.Add(new SqlParameter("@firstname", SqlDbType.VarChar, 50));
        myCommand.Parameters["@firstname"].Value = txtFirstName.Text;

        myCommand.Parameters.Add(new SqlParameter("@lastname", SqlDbType.VarChar, 50));
        myCommand.Parameters["@lastname"].Value = txtLastName.Text;

        myCommand.Parameters.Add(new SqlParameter("@office", SqlDbType.VarChar, 75));
        myCommand.Parameters["@office"].Value = dwnOffice.SelectedValue;

        myCommand.Parameters.Add(new SqlParameter("@team", SqlDbType.VarChar, 20));
        myCommand.Parameters["@team"].Value = dwnTeam.SelectedValue;

        myCommand.Connection.Open();

            myCommand.ExecuteNonQuery();

 Do I run a completey different select command before hand and try to match that field?