Maxperson
Morkus from Orkus
DJ overhears Finn talking with Poe via communicator, who explains the transport escape plan, just prior to their capture. As for how the FO figure out the cloaks, presumably the just *do*, once alerted to the plan. It's not the transport cloaks are presented as revolutionary new technology, more like a desperate trick. Note this is a sensor cloak only - the transports are visible to the naked eye - they could have used optical telescopes.
If it were visual only, the First Order would not have been able to hit the transports as reliably as they were. It's not as if their canons had scopes on them. Somehow they were able to target the fleeing rebels with sensors.
Ever notice how space combat in the Star Wars universe all takes place within the frame? Within visual range. The smaller craft behave like WW2 planes and the bigger craft like ocean-going frigates in a pirate movie. These depictions are governed by visual -- cinematic -- logic, not physics.
Ever single space battle shown in the films is nonsensical from a physics perspective. Ever single one. Search your feelings; you know it to be true.
Not necessarily.
http://www.dailymail.co.uk/sciencetech/article-2056018/Laws-physics-change-depending-universe.html