Just wondering, how does one check to see that the hanger is aligned? My cats dropped my bike onto its right side. Is there any sure way to check that the hanger is aligned? Eyeballing it, it looks fine, and shifts fine. Is that all I should worry about?
If it looks fine and shifts fine, don't worry about it. Shifting fine is what we're after.
If it starts doing goofy things, like refusing to shift into a cog in the middle of the cassette, you'll probably need to have a shop use a gauge to realign your hanger. As rear cassette spacing has gotten closer, hanger alignment has become more critical. I think that the best way to eyeball it is to shift into a gear combination that has the derailleur cage pointing straight down and looking at the bike from the back to see if the cage is parallel to the main frame triangle. Unless you're really good, that might not be accurate enough.
You would have to be pretty good to eyeball it. I just finished restoring an old Motobecane. The hanger was visibly bent. I crowbarred it to where I thought it should be and the derailleur worked reasonably well. I finally broke down and bought an alignment gauge and discovered that where I thought it should be was close but not right on. The derailleur purrs like a kitten now. I'd go with the original suggestion that if it purrs like a kitten, it is good.