I mean, I can imagine very well that you can just tell from their emotions.

Like, let's say this was their emotion timeline without superhero costume:

🧍‍♂️😃🙂😮‍💨❌

If they'd just generally be in a better mood, the emotion timeline would look like this:

🦸😁😄😃🙂😮‍💨❌

So, their mood starts higher, but there's the same rate of decline.
Whereas, if they persevere, it's more like this:

🦸😃🙂😮‍💨😔😑❌

So, similar starting mood as without costume, but the decline is only at the start until they hit the point where they would normally give up.
And then they muster the perseverance to continue, despite being in a bad mood from that point on.

Obviously, you'd need to formalize that, and not every kid will show the same emotions. But when averaged, I do think that gives you a scientific result.