What makes distinguishes something to be a correct form of the future tense?
What distingushes something to make it the correct form of the future tense? For example: Blank Uds. por avión? (Will travel)
You could add the future endings to the infinitive (see the grammar section in the reference site of Spanishdict.
You can also use ir a + infinitive.
¿Van a viajar por avión ustedes?
Are you going to travel by plane?