The resul ts are there to see. Take Toba for example: it deposited some 600 cubic kilometers of ash. Locations as far away as India were covered 6 feet deep. You can still see the ash layers.
Like I say, the evidence is quite obvious to anyone willing to look. But not to those with eyes pressed tight shut.
Still tricky to say “the evidence”. It’s unscientific to look at physical material and backstory it into being “evidence”. Nobody was around to observe Toba either.