How can you check the reputation of a private investigator?

Have him/her investigated by another private eye.

There's always your local Better Business Bureau, or the PI's union:
http://www.ipiu.org/. A little web searching turns up that most states
have their own version of the PI's union: the British one is at
http://www.theabi.org.uk/, for instance...

Hope this helps,

Thanks Rogerwilco! I didn't know there was a PI's union.

All but 6 states in the USA require private investigators to be
licensed. You should first make sure the PI you want to hire is
licensed, bonded, and insured. Contact the licensing authority in your
state, which is usually the State Police or the Secretary of State. A
complete list of these authorities are available at
http://www.pimagazine.com/links_Licensing.htm  It would also be
advantageous for you to select an investigator who is a member of
their state investigators association. This would help to insure that
they are up to date on the latest laws concerning privacy and
stalking. A list of these associations can be found at
http://www.pimagazine.com/links_State_Reg.htm It is also wise to
contact the Better Business Bureau for the area that the investigator
has his/her business to see if there are any complaints.

The Better Business Bureau is a good place to start.  Also most states
have associations which you should be able to locate online.  You may
also want to contact the state licensing authority to see if there has
been any investigations or discipline.  Did you have a problem with a
PI? If you are just checking prior to hiring (which is a good
practice) make sure you ask for their experience.  I would use someone
with considerable law enforcement experience.
http://www.PrivateEyeInvestigations.com

There is not actualy a "union" for PI's, it is just another association.

Great comments! Thank you all so much. I haven't had a problem with a
PI in the past, I'm just checking prior to hiring. I now feel much
more prepared to start looking! Thanks again.

severisth - I am actually a private investigator with one of the
largest firms (although we only work for companies, not private
individuals - I am assuming your question relates to a personal
interest, correct me if I am wrong). It can be very difficult to check
the reputation of a PI. Most PIs' work is done confidentially, so
asking for references isn't always productive (although ask anyway!).
Equally, a PI that is very well qualified in intellectual property
issues (counterfeiting) may not be any good for insurance claim
verification, for example. The absence of effective systems of
oversight and monitoring is one of the principal barriers to the
development of the sector, in my opinion.

You should also ensure that your PI has a good understanding of any
ethical issues that may be involved. There are, unfortunately, many
PIs who act in an unethical manner in the belief they are doing their
clients a favour. If the purpose is to gather evidence for use in
court (criminal or civil) gathering it in the wrong way could
prejudice your entire venture.

If you are in the US, I would suggest that you consider looking for
PIs that are licensed by the state (obviously!), but also
well-established and members of recognised industry bodies like ASIS
(http://www.asisonline.org/) or Association of Certified Fraud
Examiners (www.cfeonline.com). However, even these are no guarantee of
success, as entry requirements are not that taxing. If you know any
lawyers (esp with experience of employment law) or insurance
executives, you might try asking them if they can recommend someone.

"I would use someone with considerable law enforcement experience."

I wouldn't, necessarily. It depends on the nature of the task. Former
police officers (in my experience) can often have a poor understanding
of commercial issues in the context of a complex fraud or
pre-transaction due diligence. On the other hand, if you are looking
to recover property or perform surveillance - it's probably as good a
place to start as any. :)

Thanks for the reply konabunny! That's great information.

Another place to check would be the National Association of
Investigative Specialists (NAIS), which publishes an online membership
directory here:
http://www.pimall.com/nais/dir.menu.html

A word of caution about the IPIU "union" mentioned in a comment above:
this company has been named at Ripoff Report as a scam. You might want
to check here:
http://www.ripoffreport.com/results.asp?q1=ALL&q4=&q6=&q3=&q2=&q7=&searchtype=0&submit2=Search%21&q5=IPIU&Search=Search

Best of luck,
Byrd-ga