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What do you mean? How you learn that one is about to happen? Normally, Mega organizers are keen to publicize their event, so if you are a member of some local forum there will probably be posts about it. If not, just make a search for Mega events using the Map Compare page once in a while. Since Mega events are created a long time ahead of time, a couple of times is probably enough. That would be something like http://project-gc.com/Tools/MapCompare?profile_name=pinkunicorn&nonefound=on&ownfound=on&hidden_fromyyyy=2015&hidden_frommm=2&hidden_fromdd=1&type%5B%5D=Giga-Event+Cache&type%5B%5D=Groundspeak+Block+Party&type%5B%5D=Mega-Event+Cache&submit=Filter (where I've added a published time filter as well to not show still-active Mega listings where the event has already happened).