"He is on the radio."
Translation:Il est à la radio.
Is there a way to know when to translate "on" as "à" instead of "sur"?
No, unfortunately. You have to learn each verb with its own construction, as you did for English. The French language was not built on the basis of any anglo-saxon root but on latin & greek roots, hence huge differences in syntax.
he "is on" the radio did not make sense to me, so i tried "il parle à la radio." no deal. how do you (not a nonsense question, really!) make a distinction between location (à la) and something like this, where the subject is speaking on t.v. or radio?
To French people "he is on the radio" would mean that the guy is seated on the radio set. This demonstrates that prepositions are whimsical and that you have to learn them as they come. There is not always a logic behind the use of one vs the other.