Any time you roll a crit, or otherwise do something worthy of it, I will tell you that you have earned a check. Keep the running list here, and we'll resolve them during winter phase.
Think of them as experience checks.
Note that a check is a flag, in that a given skill, trait or passion only gets one check. Triggering checks more than once has no benefit, as you only get one check during winter phase.
Note that not all checks are positive. Doing something that earns a Lustful check forces you to make that check whether you want it or not.