How do you tell how dirty your power is?

A different way to consider the question is 'how can you tell how robust your equipment is?'. The better your components internal power supplies are designed, the less neccessary or detectable any type of ac power fiddling will be. I really like Elizabeth's note about using lights for a cheap sensitivity test, cept I'd go for high wattage (more current draw, less resistance, more dramatic effect).

One of the worst culprits, and very easy to assess, is fluctuating line voltage. It's a serious problem if you use a tube amp with manual bias. Just stick the probes of a simple volt/ohm meter into your wall outlet at different times of the day and see what it reads. If it's always low (118-ish) or fluctuates between 118 and 123, then you're going to need a regenerator like an Exactpower. No conditioner will help, but a regenerator in addition to holding a steady 120, will do everything else a conditioner will do but without ruining dynamics, etc.
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Plugging in something like the PS Audio Harvester will give you visual confirmation that there is noise in your power supply.
If you turn off all appliaces in your home and notice that you are hearing more details which were previously absent you have audible confirmation that there is noise in your power supply.
The other audible difference is in the music which should in most recordings never all have the same volume i.e some instruments must be clearly softer than others.

If your audio system shares power with any other appliance chances are it will be "dirty". The often overlooked solution is to get a dedicated line from your distribution box to your audio system.

There are many "solutions" out there such as:
Regeneration - PS Audio
Balanced Power - BPT
Passive Filters - Shunyata
Transformers - RGPC
Using Shielded and/or Braided Power Cables

Find the ones that works for you.

Athipaul, what confirmation is there that lights flashing on the Harvester or noises coming from the Noise Sniffer show noise on your ac line? I agree with your basic advice that if you try a device and like it, why not use it. Again, apart from a dedicated line, most other corrections may harm as well as help ones sound. Often greater quiet comes with less dynamic range. Would it not be great were ones power pure 60 Hz only at 120 volts? Would it not be great were power supplies able to yield pure dc at the desired voltages regardless of what comes in on the power lines?

I say buy a PS Audio Powerplant Premier.

It has a distortion meter and gives you percentage distortion of raw power vs. after it has been regenerated. Usually around 3-5% distortion from the wall, and .3-.5% after regeneration. (one tenth of what it was)

This way you know not only how bad your power is, but what it sounds like cleaned up. PS gear has a 30 day money back, so if it's worhwhile on your system, you'll find a way to pay it off, and if the improvement was not substantial enough, send it back. All you lost was the shipping.