How do you tell how dirty your power is?

TBG makes a good point. There are several types of power filtering, from transformers to chokes to capacitors to active re-generating devices. Each has its place and results in different trade-off's. Some can work together and others can't. Some work better for source components but not for amps. There's more to this science (yes, science, because each has a provable result on dirty power sources) than just buying something and saying "it helps" or "it makes things worse".

There's a lot posted about power conditioning on many forums. Do some searching and read up a bit. Then by all means, buy something used that makes sense and try it out. But just because that may not work doesn't mean that the correct type of power conditioning isn't needed in your room.

Enjoy,
Bob

I didnt know the PS Audio product has a light that can tell you how it is working, I have a Blue Circle Noise gizmo but cant tell any benefit, thanks all.

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.
.

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.