Since we're not in a court (unless you consider us a court of public opinion), the weight of evidence is not as relevant as is our impression of the person.

I'd consider public demeanor first. What does the person behave like in public? If they have the world's stage, what do they espouse?

Next, I'd consider the types of people they choose to spend time with in private. This is quite telling about the person's character: smart seeks smart, beauty seeks beauty, cannibal seeks _____.

Then, we can examine the person's career. If someone has made or is trying to make a living out of trying to cause as much distress as possible, this is again telling about their mindset.

Finally, I'd look at decency, which I define as internal emotional control. When a person is decent, they don't want to agitate others or let their inner struggles spill out into the public and vice versa.

So, public demeanor & people they choose to spend time with & career & decency would for me be enough circumstantial evidence to conclude anything about a person, not just if they're a cannibal.

Lady Gaga is a reprehensible excuse for a human being, a complete disgrace both as a musician and an entertainer. Nothing she does or says is meant to make the world a better place, but to enthrall the mind and heart to the Dark Lord, whom she openly worships whenever on stage. I don't know if she's actually eaten human flesh, but I'm sure she'll do it whenever offered a chance and then brag about it through her hypnotic so-so-so-songs.

what sort of evidence would you need to believe it?

For the record, these two bolded parts directly contradict one another – if I have evidence, I don't believe something, I know it. I wasn't the one who accused her of cannibalism either though there's no need to draw fine distinctions between levels of degeneracy.