I would personally argue that it's a lot easier to say something definitely isn't x, with confidence, than to say it definitely is. I definitely don't know what the surface of jupiter looks like, but I can pretty confidently say it doesn't look like Kansas. I think the better it gets, the easier it will be to spot the shortcomings, because the gap between what it can do well and what it can't will widen. Anything the technology is fundamentally incapable of ever achieving will be made obvious by the fact that it will simply continue to not achieve it. We may not be able to easily define the totality of what exactly it needs to have to count as AGI, but the further it progresses, the easier it will be to point out individual things it's definitely missing.