The indispensible Freivogel on conflicts points to this opinion (737) of the New York County Lawyers' Association. The gist of it is that "dissemblance" is OK in some circumstances, so long as it does not rise to "deceit," (which, in the colloquial sense, is an indispensible part of dissemblance). The opinion has in mind "testers" checking for unlawful housing discrimination, persons posing as buyers of infringing goods, and so on.
I don't know what the opinion means by saying deceit and fraud can be distinguished from dissemblance by the "degree" of deception, though it is plainly right to distinguish between the purpose of taking money and the purpose of gathering evidence. Put slightly differently, the sort of dissemblance the opinion has in mind invokes no justifiable reliance interest in the sense that the law does not want the target of "dissemblance" to rely in the manner he or she is relying.