How to tell if tank heat pads work ?

[porthole]

I use this. Mine is a bit older and lower range, but it will read up to 400 amps DC.


https://www.amazon.com/Sperry-Instruments-DSA540A-Function-Snap-Around/dp/B00279P6C4?th=1&psc=1


Another meter i use is this one.
This is typical of a meter that the leads are put in line of the circuit to read current draw.

But many meters like this are limited to 10 amps or less

https://www.amazon.ca/Ancor-Marine-Grade-15-Function-Products/dp/B018RR9O8G