Will you take fiduciary responsibility for the plan?
A 401(k) plan is set up for the exclusive benefit of participants, who may seek redress if the plan isn't suited to their investment needs, doesn't provide the proper guidance, or performs poorly relative to the market. If your adviser doesn't assume responsibility, then the plan's sponsor -- namely your company and its officers -- is at greater risk of being held liable. In which case, find another adviser.
Do you take compensation from an investment company?
A sales commission from a plan provider may cloud your investment adviser's judgment; he or she might steer you toward a provider that may or may not be the best match for your company.
Are you certified by a professional organization?
It's no guarantee, but hiring an adviser who has been vetted by a standard-setting organization (see "Resources" in How to Launch Your Company's 401(k) Plan) increases the odds that he or she will follow best practices in maintaining your plan's integrity.