Well, you can tell if a game's framerates aren't satisfactory to you.
You should also be able to tell if a sound system's audio output is satisfactory to you as well. It's all very relative, based on expectations. As for basic acoustics, its mainly limited by physics (materials and design). 5 Year old Klipsch speakers are no worse than today's PC speakers, minus wear and tear. This is because A) PC speakers were never designed for optimized audio output and 2) PC speakers have very limited physics.
Now, if you wan't to "expand your horizons", just go bring a CD to Best Buy Magnolia or any Stereo shop and listen to the bookshelves they have. If you find that the Klipschs are not satisfactory after your demoing, then, you'll know yourself if you need something better than PC speakers.