Watch this video carefully, and at the end they have still shots where they show why the real protestors knew these were police provocateurs.
This happened in Quebec August 2007.
This next one happened in Greece June 29, 2011.
Notice how the police take these fake "protestors" that have caused the commotion aside, but teargas and attack the real protestors that didn't cause the problems.
And here's Louk the Greek protest dog, a stray that joins in every protest since 2008: