Lots of things could conceivably convince me of the existence of alien spacecraft.What would be convincing evidence (to you) in terms of proof of alien spacecrafts?
Many photographs of the same spaceship(s), from a set of unconnected and reliable sources, might do the trick, for instance. A decent-sized spaceship hovering over an Olympic stadium, with all the world's media there to observe it, would be pretty convincing to me.
Alternatively, people could produce actual examples of crashed or landed alien spacecraft. They would have to display technical features that are persuasively non-human in origin, of course. Strange engines or other propulsion hitherto unknown to humankind would be a dead giveaway, for instance. But at a minimum I'd like to see a really convincing example of a spacecraft that would be so difficult or costly for human beings to produce that the chances of it being a human construct (i.e. fake) would be negligible.
I could also be convinced indirectly of alien visitation to Earth. For example, the aliens could broadcast clear radio or TV messages widely, conveying information hitherto unknown to human beings. Or they could leave super-advanced technological gadgets in convenient locations where the general populace (including the world's reliable media sources, again) could observe and interact with them.
This is by no means an exhaustive list. Lots of things could potentially convince me about aliens visiting Earth.
I can say, however, that none of the existing evidence that UFO nuts say proves aliens currently convinces me. That's not to say that there isn't some existing evidence that I'm unaware of that might convince me. I'm open to looking at any kind of evidence.
Certainly, fuzzy photographs of unidentified objects and breathless tales of seeing lights in the sky (or whatever) aren't enough to convince me. Those things shouldn't be enough to convince any clear-thinking person.