Character is consistency across changing incentives.

Incentives change constantly, and most behaviour changes with them without anyone noticing. Character is what stays the same when the reward structure moves, when honesty stops paying or generosity stops being seen. It is only visible across changing conditions, which is why nobody can really assess it from a single situation.

Watch what happens to your standards when the incentive shifts. Notice whether you still do the thing when it stops being rewarded. That is the measurement, and you are the only one in a position to take it.