Some sort of continuity tester is what you want.
kj
I guess that could work but it would be more helpful to measure resistance.
However this would indirectly measure scion viability by measuring moisture.
The main problem would be sticking in metal probes into the plant might just kill it.
I think a non intrusive measurement would be best.
Some plants manipulate the natural electrical field around them when flowering, some scientist think this is a method of communication with insects.
This area of science is relatively new.
Experimentation is required