There are home heath test kits for potassium levels but I don't know anything about them. In the summer time when I"m working and sweating a lot I supplement potassium along with using magnesium oil. Getting muscle cramps is one way of knowing I may be getting low on those two minerals.
I should probably look in to getting a home test for potassium myself. Fatigue and muscle weakness are signs of low potassium along with muscle cramps. When I supplement potassium it's usually with pills I've gotten from Sam's club.
Hard boiled eggs are a source of both protein and potassium. When I was trying to build my strength back after my heart operation I ate a lot of them. I ate cantaloupe along with the eggs and that's another source of potassium your dad might enjoy eating.