Your board can set up an internal audit committee with one or more board members, maybe a member of the budget committee and another community member with finance/audit background. The committee could work directly with a part time paid internal auditor who reports directly to the board. The committee would also report directly to the board and need to follow all the public meetings laws. The committee and the auditor need to have a very clear charge as to 1) what they are supposed to be reviewing 2) what policies they are supposed to measure the expenses against to determine if they were appropriate 3) authority of the committee in requesting information (typically you do limit how much staff time they can burn up, and the type of information received) 4) expectations of reporting frequency, level of detail and timing.
There is nothing in the statutes that prevents the board having an internal auditor report directly to the board. For more information on the use of internal audit committees, see Mr. Trainor’s website: http://mgmtaudit.com/