Well, that is the "million dollar question" of the day. There is no real good code. Most mystery shoppers and merchandisers use one of the below:
541910 - Product and Service Evaluation
541910 - Market Research & Public Opinion Polling
541990 - All other professional, scientific, & technical services
561600 - Investigation & security services
999999 - Other/Unclassified
Any of these seem to work, and I don't know of anyone, nor have I read of anyone on any of the forums getting in trouble for using any of the above.
Pick your poison?