The simple answer is that you can't. Power lines are physical cables that interconnect machines in your office, as well as between offices. It's just not possible to identify the use of these kinds of communications without resorting to measuring noise on those lines, or (as Lucas said) buying a bunch of hardware that uses those communications technologies and hooking them up. Even then, you're going to have problems of cross-talk and interference, and a user could probably just unplug them from the socket when you're not looking. It's not feasible in the slightest.
If you want to prevent someone from using one of these ethernet-over-power technologies to communicate with sensitive equipment, install a UPS on that equipment that has conditioned output. These UPS units take an incoming AC power source, regulate it to DC via a switch-mode power supply (SMPS) and then use a large power transistor (e.g. FET or IGBT) to "step" the output power, either directly (by driving a 50/60Hz signal) or indirectly (charge pump into a capacitor). Part of this usually involves a feedback step, which allows very precise changes in output voltage to be made in order to maintain a perfect signal. The main reason for doing this is that load (i.e. the amount of current being pulled by devices on the UPS) will often alter the performance of the UPS, especially when load spikes occur. The feedback allows the UPS to maintain phase and voltage at all times. The overall process entirely removes any noise from the input supply, and makes the use of power-communications technologies nearly impossible on that circuit.
The fact is, though, that a determined person will always find a way. They could use WiFi, Bluetooth, XBee, ethernet cables, USB drives, FireWire, etc. to defeat any kind of air-gap you have installed. You need to prioritise your protection to sensitive areas, and mandate strong security practices elsewhere via policy.