There's a wonderful little body of law trying to harmonize the ethical rules — which say a lawyer can't use someone to (a) have an ex parte contact with a person who is "represented by counsel" (which is WAY broader than you might think; (b) have contact with a person who is not "represented by counsel" by appearing disinterested when he is not; or (c) engaging in deceit — with the use of undercover investigators to find out, e.g., if the target sells knock-off goods.
Boiled down, the rule seems to be developing that, if the contact is made pre-suit and without knowing that the person is "represented by counsel" (again, very broad counter-intuitive meaning), and if the private investigator only acts like an ordinary consumer, then the contact is okay. A recent case so holding is Turfgrass Group, Inc. v. Northeast La. Turf Farms, LLC 2013 WL 6145294 (W.D. La. Nov. 20, 2013).