A good way to find a normal grandmother is to come to the gym on February 14 and choose from the available ones. Because if the grandmother is in the gym on February 14, then:
1st Inf 95% that it is free (employed on this day are sitting on the coffee shops and fucking for Valentines)
2nd Infa 100% that she goes to the gym
Three Inf 90%, that it is not TP (according to the legend, free TPs on February 14 sit on the window, wrapped in a pledge, sad and think of NEM)
Try it!