Phrasal verbs are verbs that have a preposition that changes the meaning of the verb from its base form. If the verb is inseparable, it means that the verb and the preposition must be together in the sentence.
If you use a pronoun to replace the object, the pronoun must go after the verb and preposition.
Unfortunately, there is not surefire way to tell if a phrasal verb is separable or inseparable. It just takes a bit of memorization.