It seems to me that your interest is confined to whether or not the person
is competent to do the consulting, competency being a continuum of
possibilities conditioned by many things.  The way you pose the question,
availability seems to be the issue, not intellectual competence.  For
instance, if the consultancy requires two straight weeks of on-site work
during a period in which you could reasonably think that the consultant has
(primary) duties elsewhere, the statement you get from the prospective
consultant declaring that she (or he) will participate should include her
statement that she is available as specified.  I think you could obtain such
a statement without a great deal of difficulty.  If the consultancy does not
specify times or durations on site, you really do not have much reason to
suspect that the person cannot or is not allowed to do what she or he offers
to do.  As in any contract, the "offer" to do work is predicated on many
things, two of which are availability in the literal sense and in the legal
sense.

It also seems to me that institutional policies on consultation will be
difficult to enforce at the instance level.  I think it is typical that
institutions proceed against faculty members only when there is a clear and
consistent pattern which over the longer haul can be shown to materially
interfere with the employees (primary) responsibilities.  Many institutions
ask or suggest that their faculty perform consultant services in their
communities (consistant with their primary duties) as a part of the triad of
teaching, research, and service, and are silent on the matter of remuneration.
Jim

James R. Brett, Ph.D.
Director, Office of University Research
California State University, Long Beach
310-985-5314
310-985-8665 fax
xxxxxx@csulb.edu
http://www.csulb.edu/~wwwing/office.html