To search for federally registered trademarks, you can go to the USPTO.gov website, and click on "search" in the trademarks panel. That will take you to the "TESS" system.
However, in the U.S., trademark rights are derived from use, not from registration, so someone might be using the phrase as a trademark even if it doesn't show up on the USPTO.gov search engine. You can try to search more widely on your own (e.g., through Google -- not recommended if the phrase is very common, or if you don't know what you're doing), or can hire a professional search company to do a thorough search.
At this time, please note my disclaimer, below.
However, if someone is already using the ".com" domain name as a trademark, the question of whether you "can" do it becomes more an issue of whether you want to go through the very likely trouble and expense of fighting the existing trademark holder over it. Even if they have made the ".com" an important part of their trademark, like "Pets.com" did ("Pets" alone being so totally generic that they'd never be able to succeed at using it for a pet-supplies retailer's trademark), they can still fight you over "likelihood of confusion" issues (and perhaps "dilution" as well, if they have a "famous mark"), and you might be better off choosing another name rather than fighting it out with their lawyers.
"Likelihood of confusion" means that the phrase doesn't have to be an exact match, it just has to give the same impression to consumers. Using "Lindows" as an operating system name (or, more specifically, a Linux distro name) got the distro maker sued by Microsoft. Another long-running cage match is one between "The North Face" and "The South Butt" (which uses a distinctive parody of the TNF mark). (I'd thought that one was settled, but someone told me yesterday that it was still ongoing.) I vaguely recall that there were a bunch of similar disputes related to the TLD during the dot.com boom, with people trying to start "Amazon.cc" or "Amazon.net" or "Amazon.us" or "Microsoft.kz" or whatever -- you might also want to look into the UDRP and consider how that could affect you, since that's one way existing operations try to go after people who pick up similar domains.
Up to you.