how can you tell if a verb will be separable or not?

A good way to know whether a verb is separable or not is whether or not the emphasis is on the prefix or the verb stem when the infinitive is said out loud. This is a very good rule because there are no exceptions, but it does require you to know where the emphasis of the word is.
For example, the word ankommen (to arrive) is separable because the emphasis is on ANkommen, and the word bekommen (to receive) is inseparable because the emphasis is on beKOMMen
another way to know whether the verb is separable or not is to memorise which prefixes are separable and which aren't:for example all prefixed verbs beginning with 'be-', 'emp-', 'ent-', 'er-', 'ge-', 'miss-', 'ver-', 'voll-', and 'zer-' are in separable.