not only can you see which seats in the aircraft you are using have a power supply
Unfortunately you wont be able to tell whether the power to any particular equipped seat will be working until you plug something into it (see below). Seat power is not considered an essential service by airlines, so it not usual to find some seats on any flight in which the power supply is not working.
You will need a special adapter to use an Empower socket, and I got mine over the net. It's called a teleadapt and is an inflight power inverter which handles AC, DC AND has a special plug for empower. This was much cheaper than the model proposed by British Airways at the time in their 'Shopping on Board' publication.
The BA 'Shopping on Board' device is a Teleadapt inverter!
You may also have to gently educate the cabin crew about your CPAP and reassure them that your wearing a mask does not mean that you are imminently about to expire!!!
For any flight on which you intend to use an xPAP device on board, you should advise the "Special handling" depatment at the airline concerned as far as possible in advance of the flight. This will serve to alert both the ground-side and air-side crews of your need for extra carry on, a seat with working
power and the fact that you will be xPAP inflight, so everything goes as smoothly as possible.