Limiting the current into Raspberry Pi GPIO pin
just need to check that the voltage it is producing is 3.3V by connecting it's output into one of the GPIO pins of Pi.


No, that is NOT the way to check the voltage. Beside the chance of damaging the Pi, even with resistor, the Pi can not tell you what the voltage is. 

Buy a Volt meter. Nowadays you get a good universal meter for as little as $10. (I bought five** good ones for $5 each, a few years back).

If you want to continue dabbling in electronics you are going to need one.

**I gave some to family so when I visit and they ask me to 'look at my...' I have a meter to do it. I once managed to guide my parents by phone to use one and find the fault in a cable.