How do you tell how dirty your power is?

Well, the technically inclined may have an answer. My answer ain't so inclined. Buy a conditioner;used. Buy one that can be sold for what you paid for it. Try it;if you like what you hear you are moving in the right direction. Further advice: don't expect to much from a cheap conditioner.I believe they,(cheap ones) give you this/but take away that.---Over the years I have owned at least 20 conditioners. I now use the Adept Response. Used, they run around 2k.In five minutes' time you will know just how dirty/or clean your power is.

Does it sound better at different times of the day, or if more or less lights are turned on, the dishwasher, washer or dryer running?

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".