Google results, when done properly (multiple words in quotes) are a good indicator of how popular terms are.
Cheesebaseball.com is worthless because the 43 sites that mention it have the terms accidentally next to each other and not on purpose. Baseballdiamond.com is better because there are 38,700 sites mentioning it. Baseballbat.com is even better because that has 240,000 sites listing it.
Now, after that you have to figure out whether the popularity of the word or words is related to commercial possibilities or not. "Having" has 42,900,000 mentions but it's awfully vague for a site name.
Still, those things are worth weighing.