Could I in theory take a soil sample and test its PH rather than take the risk of over watering my plant to get a run off reading?
First point - Watering to runoff is not overwatering. Overwatering is failing to provide a complete dry cycle between waterings. Many growers (self included) water to runoff every watering. It helps prevent salt buildups.
That said, what you have suggested is called a slurry test, and it is an acceptable way to test pH.