It's something I've had trouble looking up, but when did everyone in the Marvel universe find out that the X-Men were still around after Fall of the Mutants? I know that it ended with them coming back with the world and their friends thinking their dead, but was there a specific storyline or event or issue where their return is made public (or at least their friends who thought they were dead), or was it never addressed?

Genosha had announced to the world that they had arrested and were putting the X-men on trial during X Tinction Agenda. So, I guess then.

You could assume that Inferno started the chain for the majority of there friends and the super community finding out. X-Factor was in contact with the FF and i wanna say the New Mutants were aware at that point as well. I'm pretty sure Excalibur stays in the dark for the longest but by the Muir Island saga and X-Men #1 I think the whole world knew that they were back

So yea pretty much sometime between Inferno and Agenda everyone close to them or in the public (except Excalibur) was aware to a degree.