It's a tough game man.
Takes some careful observation, ability to test small, and some solid creativity and brainstorming.
One of the best methods to see what's working... Is to spy on ads.
See what ads always run... There's a good chance they're profitable. (I've seen a ton of people promoting that Video Maker software for the last few days... Look for patterns like that)...
Not always though... Because some people DO lose money on ads and let it run... But it's an avenue of consideration...
Maybe someone can provide better data?
For now? Keep your eyes open.
And start small.
PS: Another hot tip if you belong to any CPA companies... Is to grill your affiliate manager like George Foreman to see what's working well on Facebook ads. If they're worth their salt, they'll know.