We bought a vintage O'Keefe & Merritt stove and I need to figure out what's going on with the ovens. I know the temps are off, and I think my first step is to see if they're hot/cold by a consistent amount (and then adjust the thermostats accordingly) or if they fluctuate (and then ?).

The burner flames are on high as the ovens heat and then back off to a lower level, so I'm thinking that the temps should vary less than an on-or-off flame, but I know there will still be some fluctuations. How often should I measure and with what devices: standard-issue oven thermometers? Infared heat gun? Thermocouple that I've read about?