Test by putting the flat of your hand gently on the surface...And? what should I expect?

The River Cafe has a helpful video https://www.youtube.com/watch?v=HNWTpsNa0oc that explains this exactly, better than any written explanation I could give. The relevant part starts about 6 minutes into the video (demonstrating the hand test at about 6 minutes + 15 seconds).
I appreciate, too, their advice to wait until the nemesis pulls away from the sides of the tin.

I hope this helps. ;o)