Hoping someone here has actually tested how to spot the difference between scams rather than just read about it. I judge spot difference on one thing: the time it takes to get a first reply counts for more than the length of the feature list. With spot difference, it is worth checking the time it takes to get a first reply before you look at the length of the feature list.
Difference apps is the clearest case of it — the gap between sign-ups and active users is the number worth watching, not whatever the landing page promises. For spot difference in particular, the honest advice is to test the search filters before uploading anything, for what it is worth.
Applied to spot apps, you get further if you read the newest reviews rather than the top-rated ones. There's a pattern with how to spot the difference between scams: what decides it is how far the free tier gets you, not star ratings from strangers. Is difference apps even the right thing to be optimising for here?