The simplest way to tell the difference is that in prepositional verbs, the preposition does not change the meaning of the verb.  Rather, it just adds to it.
ask for
laugh about
listen to

With phrasal verbs, the preposition changes the meaning of the verb
get in (usually get means to obtain or receive something, but 'in' changes that)
mix up
pass out

I hope this helps Sonia :)