To test for amps you need to connect between the red terminal and the red wire the simplest is just hook a voltmeter to the battery and see if the volts increase.
You could tap into, and split the 12V+ dc from rectifier, but.....
The charging system you want to monitor is so minimal at best it may never even move the needle, nor without perhaps some fancy rewiring ever show discharge.
Me, I just use the voltmeter built into my fishfinder.