How do you tell how dirty your power is?

Good question. I've often wondered the same thing. I've considered buying, say, the PS Audio P500, only to find that it's impact on my sound was negligible becasue I already had clean power.

Get a PS Audio Noise Harvester, they sell for $75 apiece (used). It plugs into your outlet only, no devices plug into it, and any noise in the line is used (consumed) to power a small light bulb. The more noise in the line the more it lights up so you can see when your refrigerator goes on or light dimmers are in use. The more devices you use, the better, it really lowers noise. THe noise harvester is not a meter so it will not give you a "numeric value".

Plug in several ordinary low wattage light bulbs in as many sockets as are in the circuit you use for audio.add a few cheap multi A/C adapters and insert more low wattage ordinary light bulbs. (the light bulbs tend to dampen and eliminate A/C spikes, High frequency junk.
Now if your system sounds BETTER with all these light bulbs IN.... you need power conditioning. If it doesn't change... you probably do not. If it sounds WORSE.. you need a rewiring of your A/C mains
This is a VERY cheap test. You can plug in ordinary lamos etc, as long as they are common light bulbs.. (NOT Halogen, not compact flourescent)

Would power impurities show up on an oscilloscope? Can one be rented? Are you familiar with reading and operation of the test gear?