6N137s would work, but you need to add a bridge rectifier to detect AC.
The PC814 is designed to detect AC, you need only one resistor to limit the LED current. You connect the output transistor between an Arduino input and ground with the pin set to INPUT_PULLUP.
Note however that whichever you use, the zero crossings will interrupt the signal so it will pull the input LOW for most of the AC cycle, but go HIGH at each zero crossing - you have to code to check every few milliseconds ("polling") and determine that the AC is off if the input reads HIGH for a number of successive checks.
For 16V AC I'd use a diode (half wave rectifier), smoothing cap, and fairly high value (100-200k total) voltage divider to bring the ~22V peak down to ~4-4.5V. Simple and cheap, no zero crossings to deal with, and sufficient isolation for such a low voltage.