The easy, practical approach is to record something and listen.
However, instead of setting them up in a typical stereo configuration I'd set them up so they are pointing the same direction with the capsules coincident. That way the sound reaching each microphone will be as similar as possible, making your comparative listen and visual level check for sensitivity across the two more fruitful. Another easy thing to listen for is differences in self-noise (level, 'texture', 'color'). With the gain cranked up the self-noise from both mics should be at about the same level and have the same smooth hiss without spittyness or popping.
If you want to get fancier, you can record something with the mics setup that way and compare their frequency curves. A frequency analyzer plugin (plenty of free VST analyzer plugins available) helps do that, and one that can be set to accumulate an average curve over time will make comparison easier.
With the mics setup that way you can also do the good old "null test". Record something. Make sure exactly the same amount of gain is applied to each channel by the preamp and/or recorder. Flip polarity on one channel, sum the two and see how well they cancel. The closer matched they are the more the common signal in both will cancel each other and the less sound you will hear. Listening to this 'difference signal' can be informative since you're hearing what the two recorded signals do not have in common with each other, so it can help identify the frequency regions where the two are not exactly the same. It's a good idea to listen this way while adjusting the gain of one channel, to see what maximum cancellation is that you can achieve. Doing that compensates for any differences in recording gain as well as differences in the sensitivity of the microphones. If the recording chain gains really were exactly identical, then the difference in gain required to achieve the best null (quietest summed signal result) indicates the difference in sensitivity of the two microphones.