The simple answer is that there's no clever pattern you can use. We understand phrasal verbs in context, so any attempt to take them out of context and create a little grammar rule is pointless. You simply need to accept and learn the phrasal verbs as you go. There are no shortcuts. I do get uncomfortable with texts which treat any verb+preposition as a phrasal verb, but in the case of "look after [someone/something]", the meaning is very different from "look", so I accept that as a phrasal verb. Yes, "after" is a preposition but it also affects the meaning of the verb.