The most foot-ticklish lees for me give themselves away early if I drag all ten nails slowly down their calves and say "I wonder where I should go next?" If they start shaking/groaning, it's going to be easy. Try the ankles and tops of the feet too, some are wildly ticklish there. If you have them cross their ankles, it can throw off the defenses a bit. The most consistently ticklish spots I've found have been just under the ball of the foot, and just about anywhere in the toes. Sonicare or a modified trimmer works well everywhere, actually found that a mascara brush is pretty devastating and precise toe-tickler. Acting as if I'm going to work every toe one by one can be a great mental tease.