Excellent question!
Okay, first of all, when you're actually giving the foot massage, pay attention to how they respond and remember spots they seem to like.
Special spots: not sure if it's the same for other people, but in between the toes (gross but worth it). It's more sensitive there.
There's also the narrow point just above the heel. A little above the foot, but it still counts.
The soft sole of the foot is more sensitive than the surround areas, which have calluses and shouldn't be paid much attention to . Rub the sole a little harder to relax muscles. It will hurt a little, and don't do it too much.