Professional lip readers can usually get a good handle on what managers are saying to umpires in the heat of an argument. "My word, what a dunderhead you can be, good sir!" and things like that. The rest of us have to guess.
At least, we used to have to guess. At MLB.com, there's an article about what really goes on during some of the manager/umpire confrontations:
'My pitcher is terrible. My bullpen can't get anybody out. My hitters haven't hit a ball out of the infield in three days. My clubhouse guys serve crappy food.'
There are also bits about Tommy Lasorda's appetite and Joe Madden's yearnings. Click and learn.