Just at a first glance, the only way to get a precise measurement is either with a scouter or ki sensing. The former is limited because the number doesn't tell you how that number compares to yourself unless you know what your power level is unlike with the latter where that quality is inherent.
Outside of that, just observing their movements would be a good indication. Android 18 was able to deduce that Mighty Mask was powerful just based on him swatting away fodder. Likewise, Goku and Vegeta couldn't sense Dabura but deemed that he was much weaker than them based on watching his movements.