Once a team is settled in this can be hard to measure. For me personally I was able to show mine by changing teams and helping drastically inflate overall team velocity, not just by the number of points I added, but how much more productive everyone else became. It is hard to do on an established team where you're already part of it, though, and that's a problem I don't know how to solve.
Go on vacation for a decent period of time? In finance, key employees have mandatory two-week leaves, which serve to detect embezzlement. Here we could have the same, but to detect positive influences.
