I lifted the big yellow capacitor and I've taken a clearer picture of the back.

Here the PCB, front and back, and what I've understood analyzing the circuit.
I tried to find "the end" of the rectifier, where the DC voltage is picked up, but without success.
Moreover, the IC on the PCB has nothing printed on it so I can't understand how it operate.
Basically I would like to know ...

Thanks for you reply.
It's the first time I work on a project like this, I know a few of electric theory but never done something like that so... :)
I like the option of a octocoupler but I don't know how to use it in my context.
I've mesured the voltage between anode and cathode and when off is ...

Hello there,
I would like to read an analog voltage from a circuit that is powered by different AC-DC transformer, different from the 220VAC-3.3VDC that powers my ESP: basically I would like to read the voltage of a led cathode to know if is on or not.
If I measure voltage between cathode and GND ...