By state laws and codes, engineers are limited to providing services only within their area of expertise.  For example, a licensed mechanical engineer who has been working in the field of HVAC is forbidden by state law to engineer or commission structural or architectural systems and elements. Doing so would put the public at severe risk. 
As a licensed engineer, expertise is well defined and the high level of requirements are standardized through the rigorous licensing process.   By limiting engineers to their area of expertise, owners are assured the commissioning authority is knowledgeable of the subject matter and reducing the owner's risk.
From the NCEES website:
"NCEES (the National Council of Examiners for Engineering and Surveying) is a national nonprofit organization dedicated to advancing professional licensure for engineers and surveyors. It develops, administers, and scores the examinations used for engineering and surveying licensure in the United States. It also facilitates professional mobility and promotes uniformity of the U.S. licensure processes through services for its member licensing boards and licensees. These services include the records program, study materials, credentials evaluations, exam administration, and more.
The Council’s members are the engineering and surveying licensure boards from all 50 states, the District of Columbia, Guam, Puerto Rico, and the U.S. Virgin Islands."
In general, a licensed engineer must have the following as described by NCEES
"While each state licensing board has its own laws regarding engineering licensure, there is a general four-step process for licensure candidates:
  1. Earn a degree from an ABET-accredited engineering program.
  2. Pass the FE exam.
  3. Gain acceptable work experience (typically a minimum of four years). In most cases, this must be completed under the supervision of a P.E.
  4. Pass the PE exam in the appropriate discipline."
WARNING:  Non-engineers who claim to provide commissioning services for engineered systems are in violation of state laws and codes and therefore not regulated.  Very often, they may attempt to commission engineered systems outside their limited area of expertise.  Insist on a licensed engineer with a degree and experience in the the discipline of the systems you want them to commission.
Posted by David G. Venters, PE, CPMP
Performance Engineering Group, Inc.