Exactly ...

How much evidence? How many works? Who decides?

I try hard never to judge the heart (eternal fate) of another; it's way above my pay grade.

I will say that if I observed a life seemingly completely devoid of conduct consistent with a faith in (submission to) Jesus, I'd have a difficult time concluding that person had eternal life (That same super smart guy said, "If you love me, you will obey my commands.")

But even in that scenario, I'll leave it to God to sort out, kind of like the thief on the cross.

I might humbly propose that many of us and many churches (of all Christian denominations) spend far too much time trying to decide who is in and who is out.

It might be better for all concerned -- churches and individuals alike -- if we spent more time examining (and seeking to grow in) our own love for Jesus and graciously encouraging others to an ever-increasing pursuit of that love (and obedience - per John 14:15).