Well besides the obvious of just asking me... You can read reviews, you can read specs, you can see how other models they have faired, above all you can listen to them... It is hard to sum up what makes a subwoofer good, because lots of the qualities go hand in hand. To say that something is built better than another is easy to say but hard to specify why. I will try (these are just in relation to a sub):
Amp: rms, peak, thd, servo controlled, weight, brand, cost, to name first things that come to mind
Woofer: size, voice coil, magnet structure, to a very small extent materials used in the cone and woofer, nice sized surround, linear excursion capability
Cabinet: sturdy, well laid out rear for inputs, etc. the basics of phase, volume, internal/xternal crossover, etc., adjustable feet, weight
Misc: remote controlled volume, parametric eq, brand name, warranty, overall size and weight

Mind you that this is a list that I am just pulling out of my head, so I may have overlooked some things, if so I'll reread and add them in.

Critical listening is good, followed by observation of scientifically observed data (reviews with specs, like link i posted from hometheaterhifi.com), and if you cant tell the difference between a sub that is $4000 and $400, then you are set, because you just saved yourself $3600. i on the other hand, know what each of those sound like, so i can compare them. bring in a good cd for bass (i use rap, because i know how i remember hearing it, because i like rap), good and hard hitting and "quick" instead of "boomy" and demo them out. cut each sub up on volume until it distorts, see if that was loud enough but still sounded good. i have to go get some stuff for a cookout, which is a good thing, because i can ramble about subs :-). hope this helps. as always you should evaluate the entirety of a product, cost included, and see if it is what you want/can afford and still want. its funny to hear people say how their sub is awesome, yet they have no frame of reference to compare it to. like for example the lfm1 might be awesome for joe blow, but side by side with my subs itd be a coffee table. granted i didnt pay $549 for any of them either, but every cent i did pay is well worth it in comparison.