If you really want to hone in on the electrolyte solution strength, you should be able to use Ohm's Law: V = I x R (voltage = current x resistance).
I have never actually done this (I use the warm to the touch method described above), but it should work in theory. With alligator clips hooked to the part(s) and the sacrificial anode, and hanging in your electrolyte solution, but without the transformer hooked up, measure the resistance (Ohms) between the clip going to the parts and the clip going to the sacrificial anode.
The voltage (printed on the transformer) divided by the resistance measured will equal the current draw (amps). If the current draw calculated is greater than the capacity printed on the transformer, then your electrolyte solution needs to be diluted (dump some of it and make up with tap water). If the current draw calculated is less than the capacity printed on the transformer, then add more baking soda.
Example:
Transformer Output (printed on box): 9VDC, 800mA (equals 0.8 Amps)
Measured resistance: 5 Ohms
Calculated current at that resistance: 9 / 5 = 1.8 Amps
So, solution is too strong...dilute it a little more than half.
After dilution, measured resistance: 12 Ohms
Calculated current at new resistance: 9 / 12 = 0.75 Amps. This is less than the capacity of the transformer, so it should be okay.
Like I said, I have never really done this test, but it should work I think. If anybody knows otherwise, please correct me.