User acquisition costs are another frame for approximately the same heuristic. If software has ads in an expected place, and is selling data you expect them to sell, then you can model that as part of the cost. If, after accounting for all the costs, it looks like the software's creator is spending more on user acquisition than they should be getting back, it implies that there's another revenue stream you aren't seeing, and the fact that it's hidden from you implies that you probably wouldn't approve of it.
This post is a container for my short-form writing. See this post for meta-level discussion about shortform as an upcoming site feature.