The IRB makes a practise of randomly testing international players in both 7s and 15s international tournments. In 2011 there were just over 1700 players tested, both in and out of competition. Eight players tested positive and were sanctioned. You can see the details of the IRB program at . Most of the major national Unions (the RFU for one) will also test their players on a random basis. How effective these tests are is (and I suppose always will be) a matter of debate.
Yes and if found positive they get banned if their B sample also tests positive.