A reality check can be done before or after you become lucid. It is a way of telling you're dreaming. As for my personal self, I mostly end up doing my reality checks after I started to suspect I was dreaming. Indeed I was right, but sometimes it didn't work so I failed to become lucid.

Reality checks and such aspects of lucid dreaming is different for everyone, some may have them 70% of the time, whilst others may never remember.

So yeah, a reality check can be before and after a lucid dream, but when it's done after you become lucid, it's usually to double check or to increase lucidity.