If you buy cones, John, buy a range and bracket your target range. The popular brand is called "Norton Cones". They are highly regarded. If you search on that name you can get to their website and read their instruction pdf.
The only problem with the cones is that they are somewhat "interprative". There is a "time at temp" component to their behavior. They "slump" as a function of how long a specific cone is at temp. They are also intended for a certain rate of change (x degrees per hour). Like Arthur said, the temp is accurate and repeatable as long as conditions are repeated. Once you learn to "read" them, you can immediately tell what went on.
I did a series of test runs with a range of cones at several times and temps. Repeatabiility is excellent. Cost is low per run, so you can put target, high, and low temp cones in with your run as a verification things went ok on every run.
Personally, I would be cautious about using them to tell if your thermocouple's ok until after you are used to them in "good" conditions. In other words, don't use 'em for the first time to try and find out if things are screwed up.
