How To Display Error

 Hai,
         My condition of the program is to check whether the username is available or not. If it is available, I should insert that username into the db, if not I should display "Username already exists".
 
I've written SP for the condition, but I dunno how to display the error message in asp.net. I've my codings below:
 
DB - SP:
alter procedure insert_user

@username varchar(20)
as
if exists(select username from test where username=@username)
print 'Username already exists'
else
insert into test(username) values(@username)
 
Default.aspx.cs:
  protected void Button1_Click(object sender, EventArgs e)
{
SqlConnection insertconn = new SqlConnection("Server=achuthakrishnan;Initial Catalog=classifieds;Integrated Security=SSPI");
SqlCommand insertcmd = new SqlCommand("insert_user", insertconn);
insertconn.Open();
insertcmd.CommandType = CommandType.StoredProcedure;
insertcmd.Parameters.AddWithValue("username",TextBox1.Text);
SqlDataReader dr;
dr=cmd.ExecuteReader();
insertconn.close();
} 
 
What is the condition I should check after SqlDataReader dr? Or else, I believe I should use @@error to display error messages in asp.net. I googled a lot and couldn't find the exact solution. Can any one help me?