How do you know if you need to add a sub (without auditioning one, I mean)?

Or you can do like I did. With the do it yourself.

Buy at least a pair of 18" woofers. You will need that size of different reasons see below.

To get a better chance to be able to get definition, fastness and so on.

Go for a open baffle (OB) "enclosure" (one reason you have to go big on the woofer with inefficient enclosure). But we are after the best SQ not efficiency.. 😉

Buy a Ikea cabinet that comes with a door. Take out some on two of the sides of the door so it will fit inside of the cabinet. 
That is your front baffle, on the door make a hole for your driver.
https://www.ikea.com/se/sv/p/knoxhult-vaeggskap-med-doerr-vit-10326791/

Get creative and brace the cabinets insides with maybe something like 4 by 4 that will act as the distance inside were you fasten the front baffle (the door). The object is to make a H-frame OB.

Then you use REW, mic and DSP. 
For finding crossover point (slopes), phase (delays), level match, PEQ, custom curve and if you are a vinyl guy a rumble filter you will probably need that. 😉

Custom curve is the other reason you need to go to big drivers. To be able play louder as the frequency decreases so you compensate for your ears lower sensitivity when frequency drops. That will make your ears to experience that the frequency response is flat to 20 Hz (and beyond). (That is one example of that our hearing is not like a measurements microphone when our hearing is NOT linear).

The "down side" is that nobody else can say i have heard those and it sounds .. or the second hand market the subs are worthless when nobody knows what they get..

But do we care what others think and do we want to flip them or play music?..

Now when done put on the dark side of the moon you can thank me later, your are welcome.

You will hear new stuff on that record that you did not hear before and you thought you had fully experienced that record and knew how it sounds.. It is now a totally new experience.

You will hear a second pair of a heart beats that is lower in frequency and level. Than the "normal" heart beats are at..

So new "music" information that the artists intensional put into their tracks for you to experience. So until now in other words you have not experienced dark side of the moon fully as pink floyd intended you to do..

And that were released 1973. So the questions are.. ..how much music information have you missed out on since then.. and why you dont have a reproduction system that supports all the frequencies that your ears can hear? 🤔

When you think about it like that then the decision-making is simple..

So the best way was suggested before, get a test CD or album that plays pink noise or frequencies from zero to 20 kHz and a pretty good DB meter. have the meter at the listener’s level and measure the readings over the entire frequency range.

I do not want to be rude "the best way" and then suggesting something that were the best way at the 80-ties. 🤔

Maybe a computer with free software like REW instead of a CD. And a inexpensive AND calibrated measurement microphone ~70-100$ much better than any DB meter..

But a dB meter is better than nothing and it is almost as nothing also in comparison. But I understand that not everyone are handy with computers and software. (No pain no gain.)

You will get graphs with levels and frequency at any point you desire in your room and move your speakers and see exactly what happens for each change you make.

You also get so much more for example can get help to find crossover point and delays for your subwoofers to time align them with your mains automatically generated.

You can do multipoint measurements and also do moving microphone measurements and you can see decay times for across the whole frequency band to see what your treatment do for you... And so much more that you can’t do at all with a DB meter.. so much more so it is not even funny. 😉💖

And remember our hearing is NOT the same as a flat graph.✋ Even if we would like it to be.. it is convenient to think so. This is regarding to get something flat.. it LOOKS nice with a flat line.. anyway microphones measure accurately.. let me explain below.

Study this graphs that show how our hearing works across the frequency band. Note that they are far from horizontal or linear/flat for that matter..
https://en.m.wikipedia.org/wiki/File:Lindos1.svg

If we measure with a microphone the same shape as the graph shows sound coming from a speaker. Then we would experience that speaker as truly flat. With equal loudness from 20 to 20000 Hz.

So as you see in the graph, if we follow the 60 line that is 60 dB at 1 kHz so for us humans we perceive that at 20 Hz at 107 dB is AS LOUD as 60 dB @1 kHz.
Because our hearing is less sensitive at 20 Hz.👂

In other words if you strive for as a goal to make your frequency response flat so you have 60 dB also at 20 Hz then you precive when you listo, that you have attenuated the level at 20 Hz by 47 dB!

And that in it self make us to not be able to hear ANYTHING at all at 60 dB @20Hz! 😱
When you can clearly see that 60 dB @20 Hz is BELOW your hearing threshold (the other read line named the same).

I hope this is food for thoughts. 🤔😍🌷❤️

(This is just science worked on since 1933 and then reassessment and revised in 1956, which became the basis for an ISO 226 standard later on.)