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

Two potential issues with boosting bass levels alone versus adding a good powered sub and setting it up properly:

1) You might increase bass SPL FBOFW but not at the lowest octaves down to 20hz which is likely what is mostly missing to start in most cases. That depends on speakers and room size/acoustics.

2) increasing bass levels will make your existing amp work harder. The lower the frequency, the harder it will work. It may or may not be up to the task so results could well be far from optimal, possibly worse overall. Powered subs deliver a lot of power typically for a reason.....it is needed for low bass. Power demands increase exponentially as frequency decreases. A powered sub will truly supplement your system’s bass capabilities. How well depends on picking the right sub up to the task with your mains and in your room and then setting up up well, which is not hard. Setting up well requires only a free sound level app on a smartphone and a source of white noise which can be streamed from sources over the internet. Or you can do it by ear which is not as reliable for best results but may still deliver the results you are looking for.

First of all, a lot of bass in some music is entirely unrealistic.  I've been with many people listening to recorded music where some said bass was great and others including myself said the bass was over emphasized.

Way too much.

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.

See where the gaps, valley, peaks are.  This also helps tell you the room effects and helps solve room issues.

if your system's bass response is what it is suppose to be, IOW, is it flat?, then no, you do not need subs at all.  your system and speakers are doing their job.

If it isn't flat, that does not automatically mean you need subs, it may be the room.  Or a combination of room and system.  

It is definitely not a simple as "I need subs".

A test CD and a sound level meter are not expensive.

enjoy

An interesting test is to do the measurements in a system that has subs with the test CD and sound level meter and see if it shows too much bass.

enjoy

Minorl: I like your idea of testing DB at all the frequencies. Aside from my wondering about subs, I'm sure I have room problems I don't know about since I have almost no points of reference as to how rooms/systems without problems sound.
It's certainly possible (and maybe probable) that I don't need anything more than what I've got, given that I can maximize/improve without adding devices or components.

By the way, I'm pleased that so many posters are really trying to help me vs promote other agendas or make me feel like a dumbass.

Do the measuring with a sound meter app and white noise to see if/what you are missing or not. Only then will you know for sure. Having that lowest octave reasonably flat in response along with all the rest makes a significant listening difference ranging from huge on many recordings to minor or even none on others.

Room acoustics is a different issue and greatly impacts bass. Not a bad idea to get a handle on that first with what you have before adding anything.