This is a very open question, but in short, you need to measure the phase lead/lag between voltage and current (0° phase lag = Power Factor of 1, etc). Assuming you're using one phase ~120VAC, you'll need to drop the voltage down with a resistor divider, then sense the current using a low impedance (ex: .01Ω, .1Ω, etc) series current resistor... actual impedance is a function of the current you're pulling: you want the voltage across the resistor under maximum current to be close to (but less than) the maximum input voltage of the analog input channel. Then you'll need to plug these low voltage measurements into two channels on the Elvis board. After that, you'll need to acquire the data and then perform your calculations.
In short, yes, it is possible to measure power factor on the ELVIS platform. Please let us know if you have any specific questions.