Way to measure AC power in 2 wire cable
Power is v(t)\$\times\$i(t) which is (voltage \$\times\$ current), so if you want to know the power you need both voltage and current. The instantaneous power generally varies with time and may flow in either or both directions (one at a time, obviously, but it may flow back and forth over an AC cycle, netting out in either direction or even at zero if the load is inductive or capacitive). 

Usually people are interested in mean power, which represents energy flow per unit time. 

So you need to know the voltage (across the wires) and the current (through at least one wire if you can assume there is no other path for current to flow). The best method will depend on whether it's 15kVAC at high current or microamperes at a volt, but generally direct connection to the wires (for voltage measurement), and either separation of them magnetically (to allow a current transformer to work) or insertion of a known resistance "shunt" (for current measurement) would be best.