How to check result set empty or null in linq????

Question

hey frnds ...

i have a query in linq like this.

  var res = from UM in linq.AG_User_Masters

                      where UM.Login_id == txt_loginname.Text

                      select UM;

now  i have to check that res contain any result or not..

but i do not getting this abjectly...

so pls help...

Thanks....