Ask a million questions.
Then ask some more. The legitimate companies will be ecstatic to explain their mission statement. If you dedicated your life to the pursuit of ecological preservation, wouldn’t you be excited too?! The companies who are up to no good will get irritated- fast. Some typical questions you should ask are who owns the company, how was the company started, and how do they measure impact?
If a company claims non-profit status, make them verify it.
Ask for their tax info or search them on guidestar, one of the leaders in NPO verification. Generally, it’s illegal to say you are a NPO (non-profit organization) without actually being one. In the states, when an organization claims 501(c)3 status, they must make all tax and financial information available. If they are unable to provide or verify NPO status, you can pretty much assume they are full of shit.
Research any certification claims.
Remember all that time you spent looking into hotels only to find out they were full of shit? What is the point? Try to at least make it difficult for companies to greenwash by taking a few minutes to research. All you have to do is google whatever certification they have. If nothing immediately pops up, on google, you can guess that the certification is illegitimate.
Ask for testimonials.
Especially for long-term trips! If you plan on trying to volunteer with an organization for longer than a day, you really should consider obtaining testimonials. A legitimate company will be able to not only provide you quotes, but connect you with former volunteers/participants.
If there is a fee, compare with similar programs in your country of choice.
Be extremely wary of a costly volunteer trip or ecolodge! To be charged $100 USD for a proper recylcing program is reasonable, but $400 USD a night? Probably trying to cash in on the green business.
If something seems amiss, speak up.
This especially applies to any encounter with children or animals. If an elephant sanctuary claims to be stress-free for the animals, and the workers use bull hooks, cry foul. Scream foul. Let everyone know! Why not? People who work in conservations have to fight for very limited funds and resources- save them for those who actually give a damn. Remember, when it comes to equality, there is no such thing as complaining!