Other than memorization, you can think about whether or not the particle makes sense as a preposition. Find a good list that indicates the kind of verb, and also whether or not -- and how -- the verb may be separable. Also, if it's a true phrasal verb, the accent will always be on the particle, but if it's a prepositional verb, the accent will usually (though not always) be on the verb, not on the preposition. In addition, verbs that are prepositional in English tend to be so in Spanish, whereas English phrasal verbs often utilize prefixes in their Spanish counterparts. I hope this rather brief answer proves useful.

To be honest, Ana Pau, it doesn't matter too much what you call these constructions... phrasal verbs, prepositional verbs, phrasal-prepositional verbs. The important thing is to know what these 'multi-word verbs' mean,how they fit into the sentence, and whether or not you can separate them. 99% of native English speakers have absolutely no idea what a prepositional verb is - but everyone can use and understand them. Don't worry about the terminology - concentrate on the meaning and the use.

I assume you mean a prepositional phrase, not a "prepositional verb". I recommend re-arranging a sentence by placing the prepositional phrase in a different place. If the result seems non-sensical, then it could be a phrasal verb+object.

It's very easy Prepositional Verb = Preposition + Verb (talk with) Phrasal Verb = Adverb + Verb (look for)