HOW ONLINE GAMBLERS SET LINE ON HOLLYWOOD - NY Post

Basically they google it and guess:

First, they scour the most popular entertainment Web sites, reading every fragment on the candidates' behavior.

Next, they do a Google search to scratch up more information and to try to get a feel for whether the information on each of the potential fathers is positive or negative, plausible or implausible.

Third, they do what's called a "blog pulse," seeing what those in the blogosphere are writing and what they might believe.