Let's say I register dataminer.com (only an example), and the term "dataminer" is not trademarked anywhere in the world, will that prevent someone from being able to trademark that term in the future? I guess what I'm asking is, is registering the .com an effective tactic to prevent the trademarking of a name?
IANAL, but no. They can still trademark the name but they would not be able to "reverse hijack" and take the domain away from you. If they applied for a trademark, and you wanted to prevent it, you would have to hire a lawyer during the application-review period and file an objection at the USPTO. www.uspto.gov An $8 domain registration is not a short cut and does not automatically stop a $x,xxx-$xx,xxx trademark registration.
While there are online search engines to give clues, there's no 100% surefire way of telling whether someone has established trademark rights to a term. But as tobycoke answered, registering a domain name doesn't stop someone from establishing trademark rights to the term. A domain and a trademark are 2 different bananas.