Getting back to the question at hand, the answer is easy. If one of Michael's people comes to him telling him that Barzini wants a meeting on his turf for safety reasons, then it is clear the bearer of the news is a traitor.

As we know, negotiations between rivals within the mob have their own set of rules, the main one being that the "negotiators" ... i.e. the family whose name I forget, acts as the hostages while a meeting between adversaries takes place. This is the only way real negotiations take place...thats what they did when Michael was sent to meet Tatt.

For a capo to come to the don and inform him he has made the arrangements is not the way its done. Even Tom, who did not always sniff things out knew something was afoot when he asked Michael at the Vito's funeral about whether he knew "how they're going to come at you?" All Michael had to say the mettion was on Tessio's ground "where I'll be safe," and Tom understood right away by confessing he always thought it would be Clemenza, not Tessio.