Probably the easiest way is to go to the Marriott website, use the "HELP" function to search for WiFi and find the three different ways you can locate what WiFi services any given property offers and at what, if any, cost. Took 30 seconds.

As to access to event & convention networks, that is up to the event organizer. Some pay for it and make it available to all registered attendees and some do not. Presumably you will find that in the event organizer's materials.