Awesome, thank you. This with @bluejets suggestions help a lot.
As the tank will be filling (slowly) when the triggering occurs the value might drift depending on the flow rate IN but I can take the average flow rate IN and calculate that based off the scale reading.
It also means I don't really need to measure the large holding tank as @bluejets suggested.
@bluejets quick update with some more questions
So I went down the rabbit hole of getting one of these ...
All I really need to know is when is the pump ON/OFF. This will help me do a bunch of things like measure liquid movement / detect if the pump has failed to self prime as it will stay on but no weight will change on the scale reading.
I've found what I think is the circuit of this devices but I'm struggling to understand how to interface with it. I thought I could potentially create a voltage divider, to get it to 3v3, splintering off the relay coil/green led but I'm not sure how to proceed with the schematics.
Im not sure if I'm overcomplicating this now but that would be great.
[EDIT] : Im conscience that I didn't ask any specific questions above so I'll try be specific.
So pop a relay coil of the same voltage across the motor wiring (parallel) and use the voltage free contacts to do whatever you like....simple.
Sorry for my ignorance but I'm not following what you mean. Are suggesting putting a relay on the AC motor side to understand when it's triggered or adding another relay so it would effectible be a DPDT relay and use the other rail with the MCU to get a read of the state?
I thought there would be an easier way to see when the relay was ON with a voltage divider but im new to this so my knowledge gaps get in the way.
the suggestion is that if you want to know when the motor is actually powered, you connect a relay device across the terminals to the motor.
If you want to know if the pump is actually running, then you would need some sensor to detect its actual output. That is of course, a different problem. 
So the voltage monitor could be a relay rated for the particular mains voltage. Or it could be an isolated monitor module like this:
Absolutely not!
Awesome, thanks for clarifying, the relay idea is what I thought @bluejets meant but wasn't sure.
The answer still might be the same but I didn't mean off the AC side but from a similar area where the green LED is (image below) . If they shared the same wallwart with common grounds I thought it was doable. Like I said that might be way off but thought I would clarify what I meant.
So add K2 relay coil across K1 coil....as long as the transistor can handle the extra load.
Same same.....
Sorry could you elaborate? I'm not sure what you mean by this.
I ended up soldering two wires off the back to get an ON signal. Seems to do the trick. 10.5v when on and voltage divider outputs 3.3.
Any issues doing it like this?
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