OK, some will call this a cheat tip. I will probably have a lot of that going on here!
Whenever I swatch a new yarn, I ALWAYS had my friend (and knitting guru) Tracy measure it for me. See, I know me. If it doesn't match the count I need, I will either futz it into the proper count or I'll just shrug and say, "Close enough". Even if it's something important, like an item of clothing I actually want to fit on my body and look good!
With the objectivity of a friend, or LYS staffer, you can't cheat yourself out of doing a proper swatch.
So I guess this tip is an uncheat tip!