Ok, I found it; the thread was started by 'halo' on December 10, 2007. And speckledhen posted about the salt&water test.
Also interesting, from another thread, is this post by PoisonIvy:
"I did the homemade salt water test and it made mine have reading of 9% & 8% off. I then went and bought the humidity calibration tests bags from a cigar store. I left them in for 8 hours then 12 hours and took readings. It read correctly at 75%! I then tested all of them 6 different times in a 2 week period and all but one of my hygrometers read correctly at 75%. So I recalibrated my r-com 20 and this hatch the chicks popped out clean and with no trouble. I guess when I did the homemade test I didn't do something right and I then caibrated my incubator wrong. I just wanted to let you know to be careful if you made your salt water bag to test with. I would try to find a cigar store and buy one of their bags they run about $3. I bought mine offline when I ordered my temp/hygrometer from them. It made a big difference in my hatch."
Hope this is helpful!