Many companies also belong to statewide home inspection industry associations. As a trade association, ASHI sets certain standards of practice, including what constitutes a thorough inspection, and enforces those standards along with a code of ethics for its members. It also sponsors continuing education. Although ASHI membership is a positive credential, membership alone does not make a good inspector. And don't necessarily turn a non-member away if they come highly recommended.
Many worthy inspectors have yet to discover the cost-benefits of trade guild membership.
It's a common trait in the small business world. This is critical. Nothing beats experience in the field. It means the inspector has pleased enough customers to stay in business, and has encountered and dealt with a variety of home types and conditions. Don't just ask for references, call the persons listed and ask them very pointed questions.