So how do you know if a hotel is really committed to sustainable tourism, or if it's just jumping on the marketing bandwagon?

It can be tough. Not all "green" hotels participate in any kind of association or certification, and some certifications are mostly voluntary -- meaning nobody's really checking on whether the hotel's good intentions are translating into action.

Resources for researching hotel sustainability, however, are increasing.

www.greenkeyglobal.com: This Canadian-based organization rates participating hotels on their operational efficiency and sustainable practices. The most sustainable properties get a five-key rating. About 20 percent of hotels are inspected annually to check on their compliance. On the website, consumers can search by location for a rated hotel. Choices are limited in the United States. But there are hundreds to choose from in Canada, and Green Key is rapidly expanding its presence in the U.S. market, so choices should expand.

www.ahla.com: The American Hotel & Lodging Association, which encourages its members to adopt environmentally friendly practices and operates a Green Resource Center to give them assistance in doing so, plans to begin listing its member hotels with third-party green certification. The directory should go live in the fall, says Pat Maher, a hotel consultant who's also the association's "green guru" and advises members on sustainable practices.

www.greenhotels.com: The Green Hotels Association lists about 300 U.S. lodgings, and several around the world, that have joined the organization and are committed to sustainable practices. The list is searchable by state. The association lists guidelines for hotels to practice, such as using nontoxic cleaning products, recycled paper products and 100 percent organic cotton linens, but does not do on-site inspections to gauge the level of a hotel's sustainable practices. Many of its members have been recognized by other sources as sustainable.

www.travelocity.com: Search the online booking agency's green hotel directory, with hundreds of hotels around the world that have received "green" accreditation through one of a half-dozen third-party groups, including Green Key Global.