SE's ab ability to detect RMT involvment has skyrocketed since FFXI. Back then they couldn't intercept the tells and monitor the mail. Now they can. It's not that hard to see when a player has used the RMT's really because now all they have to do is look at logs.

So you answer to #2 would logically be, Who's buying a house on day one? Review the logs to see if it's all legit, or if a bunch had a sudden influx of gil that came from other Player? If so, delve into the chat logs, and viola. They'll see the RMT involvment. WHole think would take less than 5 minutes to see.