I just found the fellow I was looking for in DeKalb County, Missouri with a method I thought I'd share with you all.
It turns out "DeKalb" is spelled like that in 1850, 1860 and 1880, but "De Kalb", with a space, in 1870.
I thought at first my George Washington Kearns had moved for 1870, or had been listed as G W, or had the initial "K" in his surname indexed as an "H". I've had problems like that before.
But, I had my suspicions, so I looked for anyone named "Smith" in DeKalb County in 1870. No records. That told me I was probably spelling the county name wrong. I was – for 1870. When I tried the Smith family in "De Kalb" in 1870, I got lots.
So, if you don’t find who you are looking for when you specify a county, try looking for the Smiths. That will tell you, usually, that the county existed in the census year and you are spelling it properly.