To an extent I would say you can tell a lot of the time, especially if you are in nice and close. But they definitely call it on some that I think look very much like tackler is still too involved. If player is 'milking' to show they're still being held down then it should be a penalty regardless of the milking.
Unless attacker is interfering with defender getting off (which does happen) then milking is irrelevant, they are either held down too long or they aren't.
Some players like Woods loves a milk so got it called on him a bit too easily, and then old mate Fifita who gets called by name out of loads of tackles wayyy to early.
