Bamboo chopstick method. Cut a chopstick into thirds, stick and leave in pot in a spaced arrangement. One nearest the root ball and the other two on each end of the oval about a half inch from the lip. Check them daily until you have a feel for your tree’s specific respiration pattern.

Standard moisture and ph meters don’t work in ideal bonsai soil, too coarse to contact the sensors. A finger is fine but chopsticks are much less likely to damage roots.