Simple: Disposable items are yours to keep.. In particular, if there's a little amenity bag with earplugs, sleeping mask, socks etc handed out, the contents are all fair game, and in business class and above these can be quite elaborate (name-brand cosmetics etc). You're also welcome to take the in-flight magazine, and it will often say so on the cover.
In addition, while the airline won't care if you take home food items, on international flights Customs at your destination may object, particularly to fresh fruit etc. Stricter countries like Australia will explicitly instruct you to not bring any food from the plane. Squirreling away any unopened alcohol is also usually against licensing rules, but typically crew open the containers before handing them over.
What's notnot OK is everything elseeverything else: blankets, pillows, reusable (metal) cutlery, big bottles from the bathroom, etc. Headphones are a bit of a grey zone, and some airlines are OK with you keeping the really cheap/crappy earbud type, but if there's staff going around collecting them or giving instructions on where to leave them, that's a pretty obvious clue that you should not take them.