@jmo: sure, I'll expound on my "intuition" that I use to determine who is lying or telling the truth. When I talk to people, each person gets an individualized message. I try to ask their thoughts and determine what is going on around the board. I want everyone to get the feeling from me that I want to ally with them, because it's true. In 1901, I want an ally, and it doesn't matter who, in particular. From those I'm not neighbored to, I try to form a positive relationship. Talk about the game, talk about fluff, share information that is vital to my success. Anything I can talk about that is productive in establishing a positive relationship counts.
Once I start hearing back from others, I compare notes. A common example is when I get press from one of my neighbors that is gung-ho about being an ally. They map out detailed plans and want to start out aggressive against another player. I'm fine with that, but I want to see what others around me are getting from that same player.
--Are they also receiving similar press? If so, that means that his offer to me is not a genuine thing, merely a macro he uses with all players.
--Is his press with others different, and more cordial? If so, then it means that their press to me is genuine and more believable.
I also compare what he is saying to me with others around the map. Does my information from other sources back up what I'm hearing from him or does it conflict?
I don't usually use one single source as a way of defining whether a player is telling me the truth or not because that one player may be deceptive. If, however, I'm hearing similar things from multiple players, it lends more credibility to the truth of the statements.