The section of my Black Hole FAQ on the observational evidence for black holes is sadly out of date, although the rest of it is still reasonably current. I don’t have any plans to update it, because that sounds altogether too much like work. I’d have to read a lot about things I don’t know much about in order to get up to speed on the subject, and if I’m going to do that, I think it’d be more fun to do it on some new subject rather than revisiting this one.
If I were going to write about this subject, though, I’d certainly want to talk about some recent results published in Nature concerning observations of the black hole at the center of our Galaxy. I think you need to be a subscriber to see the article or Nature’s newsy description of it, but there’s a Science News article that I think is publicly available. (Thanks to my brother Andy for pointing this out to me.)
There’s no way to see past the horizon of a black hole, so the name of the game in this business is to try to see as close as you can to the horizon. If you can resolve details near the horizon, you can look for distorting effects due to gravity, which provide pretty definite evidence that what you’re looking at really is a black hole. If all you can see is stuff that’s 1000 times bigger than the horizon, then it’s hard to tell the difference between a black hole and any other object of the same mass. The authors of this paper have managed to resolve structures that are just about the same size as the horizon.
By the way, one of the authors was a friend of mine in graduate school. Among his lesser-known accomplishments is writing a brochure describing the Berkeley astronomy department to incoming graduate students, in the form of Allen Ginsberg’s “Howl”. I don’t know if copies of it survive, unfortunately.