What could I use to detect a leaking riser pipe in my apartment?
This is almost a shopping question.
 Molecular sieve desiccant, the indicating type would be a non electronic method. This material you could put down kinda like round BB’s or small cylinder shapes . when it turns color Usually from white to blue it’s damp, over time the desiccant will turn colors and it can be put in the oven and bake the moisture out. Many years ago I saw a metal gauge with a disk of desiccant for this exact purpose but did not see one of those with a quick internet search. The  Molecular sieve desiccant can be dried many times. Other than that electronic moisture detecting pads would be the only way I would know how to know at a glance.