Assuming that it has a standard tip and sleeve jack, the sleeve is probably grounded and the tip is the active input. Easy way to test is to open the diagnostic screen (assuming it's available in the KX3 screen set), and look for the "digitise" light while you ground the probe input - it should go green when touched to ground. Probes / edge finders are easy to make - see here;

https://www.model-engineer.co.uk/alb...47864&p=794324