how do I check if an item exists before inserting into DB

Try it like this:

public bool projID()
{
    bool alreadyExists = false;

    using (var cnn = new SqlConnection(ConfigurationManager.ConnectionStrings["ProjectsAndTasksTestConnectionString"].ConnectionString))
    {
       cnn.Open();
       string query = "Select ProjectdName FROM Projects WHERE ProjectName= '" + txtProjectName.Text + "'";
       SqlCommand cmd = new SqlCommand(query, cnn);
       SqlDataReader dr = cmd.ExecuteReader(); // EDITED
       if(dr.HasRows)
       { 
          ClientScript.RegisterStartupScript(this.GetType(), "insert-fail", "alert('Project Name Already Exists');", true);
          alreadyExists = true;
       }
       dr.Close();
    } 
    return alreadyExists;
}