We eat out quite often, and I'm still gauging the hidden carbs in alien food (food I haven't prepared). So I like to take a before and after for comparison.
If I remembered, I would test in the car, before going into the eatery, but I often forget. Too focused on stuffing my face! When this happens, I pop my handbag onto the table, or my lap, between me and the room, rummage for test kit, pull it out. Test, and put it away.
No one notices.
I've even had a waitress come over, take the order and disappear while I was doing it. Her eyes never even flicked towards my hands!
The trick is to act as if you are doing nothing more exciting than getting out your reading glasses, or checking the time on your phone. People eyes are only drawn to furtive, self conscious behaviour.
It is even better if you carry on talking naturally throughout. Since I rarely stop talking, this is very easy for me.
I would never test in a public bathroom. Or make an issue out of it. Or try and hide it.