I'm talking about the $7 glass thermos at your local homebrew store. Has anyone experienced these not being accurate? Do we assume they are always accurate forever? I know I know, I need a thermopen.
I'd be more worried about breaking it near or in the brew process. It's a no-no. Accuracy can be tested by taking two set values. Boiling water would be 212F and Freezing would be 32F. The ice test takes longer to read accurately. Lots of ice, just a little water. Watch your altitude.
Like inchrisin says, check it against the two reference temperatures, adjusting for altitude as necessary.
FWIW I somehow acquired two of the $7 glass thermometers, so I use both on brew day. I'll stick both in the mash, water, wort, etc. and then take an average of the two. Has always worked for me, although they could both be inaccurate! 2 is better than 1 though. I'd do what inchrisin says and base it off a set value, and adjust from there
Yep, duck those things, I had one break in the mash. Had to throw out $20 worth of grain, which would have almost paid for one of these: http://www.amazon.com/Lavatools-Javelin-Digital-Thermometer-Chipotle/dp/B00GRFHXVQ
