From my experience Reef-PI doesn't start when the PCA9685 option is enabled in the Reef-PI software but no PCA9685 chip is detected, so the chip appears to work (at least as soon as I remove my stacked PCA9685 module, but that should be same as with the standard PCA9685 board).
In my documentation (not released yet as I still have to build the new Reef-PI_PWM module v0.3) I've written following 3 step debugging information, should be same for the PCA9685 module:
1) Check I2C address with "i2cdetect -y 1". Required package: i2c-tools (sudo apt install i2c-tools). It should show addresses 40 and 70
2) Verify the operation of 1 port directly from the PCA9685 chip. Best is to follow a connection to a a larger resistor and measure there (I could post my details including Reef-PI setup where I setup PWM to 50% and then it should measure 2.5V, but the debugging itself refers to components on the Reef-PI_PWM board so that doesn't help here). There is 1 important note which I don't understand myself, when I set PWM to 100%, the output is suddenly 0V and not 5V ! At 99% is shows 4.99V again.
3) Verify output connector with nothing connected except multimeter (inverted 0-10V)