Some linguists claim that English doesn't have a future tense, and some do for German as well. This opinion was voiced out here as well,as an answer to What is the present tense expressing future?. I cannot determine whether this view is common or disputed, and if, to what extent.
However, if we are to assume that English doesn't have a future tense, there are arguments to support that point of view. But if it can be said that English doesn't have a future tense, one should also be able to define criteria for English (or any language) to actually have one. To rule out most obvious one - "if it is synthetic" - I'll make my question more specific:
What criteria - and why - would a syntactical construction with auxiliary verb have to meet in order to be regarded beyond (reasonable) doubt as real future tense for English? Essentially I'm asking about refutability.