If they are regular gung ho witnesses I have no trouble, the only trouble I have is when they are casual JW's that only go to a meeting here and there.
I worked with a woman who was rumored to be a JW, but it was really hard for me to figure her out, she used alot of 'worldly" expressions, plus she had multiple piercings in her ears. I did find out she was a JW only after my shift supervisor spilled the beans about me having JW family, he did it in a joking manner. She immediately said she was a JW. After talking with her I found out she was very inactive and only attends a few meetings a year. I got along fine with her, she was a nice lady even though half the shift couldnt stand her. She was one of the few people that actually showed concern for me, plus she didnt freak out when I told her I was "sort of" an apostate. She quit 2 months ago.
A 20 something unmarried male driving a 4 door car is a good clue that he is a JW.
There are many clues out there, the fun part is trying to interpret them.
When Amanda and I picketed the local kingdom hall on sunday, I made a game out of it trying to figure out which car group was JW and which car group was church people. I was seldom wrong in my selection. The JW's seemed more distant and haughty looking.