The humidity cue
If, as the popular myth has it, cabin air were endlessly recycled with negligible top up fraction from external air, on full flights it would rapidly become very humid, as the cabin is densely packed with paying living humidifiers. The portion of each exhaled breath that made it down to the alveoli has a very high dew point.
However, as is rather more generally and accurately known, the cabin air is actually very dry. So the top up from (extremely dry) external air is very substantial.
Funny that the 787 is mentioned here. I think the reference is backwards, as one 787 innovation is to _lower_ the fresh air input when the cabin has a small passenger load with a view to raising the cabin humidity a bit (mind you, still terrribly dry, at 14-15% rather than the single digits of earlier craft).