We can tell that wikileaks is now a front precisely because they have stopped operating in the way you describe.
They no longer publish everything they have been given (Assange once claimed to have documents embarassing to Russia but they are as yet unpublished).
They no longer publish documents indiscriminately (Podesta emails trickled out over the course of October 2016 for maximum political effect on the election)
If that wasn't enough, everyone should've been convinced that wikileaks is compromised when Assange refused to demonstrate that he held the wikileaks signing private key during his reddit AMA