After my experience accidentally installing an 8-speed chain on my wife's mountain bike (3 x 9 speed) last month, I checked the rest of our bike fleet yesterday to ensure they all had the correct chains. (There is no consensus on the 'net, for example, on whether it's OK to run a 9-speed chain on an 8-speed cassette. I already proven that it doesn't work to run an 8-speed chain on a 9-speed cassette.) Anyway, I dutifully recorded the various chains' model numbers and then went to the SRAM website to check out what model number corresponded to what chain type.
Part way through this exercise I realized that the SRAM 8-speed chains are all numbered PC-8xx (PC-830, PC-850, PC-870, and PC-890) and the 9-speed chains are numbered PC-9xx (PC-931, PC-951, PC-971, and PC-991). If there's anyone else out there as dense as me, I hope this saves you some time.