I would say it's very difficult. If they are with family, they'll be going thru the motions of reading etc. Even if you saw someone looking at entertainment on their tablet, they still could consider themselves a good JW.
At the place where I work, except for the JW in the office wanting to tell everyone that the world is horrible, the other two - you wouldn't be able to guess that they are JW's. But they all consider themselves to be a "good" JW.
And of course, you have to be careful about approaching someone and hinting that you might not belong to the hive.