Certification ensures that the contractor understood the study material well enough at one point in time, but how current is the contractor's knowledge of green construction best practices?
The National Association of Home Builders Certified Green Professional (CGP) certification requires their green-certified contractors to invest 12 hours of continuing education every three years in order to be eligible for renewal. Green technologies and techniques change fast, however, and a lot can happen in three years. A green contractor should constantly stay current on true state-of-the-art knowledge.
Asking a prospective green contractor about the last class he took is a good way to gauge how up-to-date the contractor remains on green design and construction.