how do i measure operating temperature
Unless your IC is one of those super complex ones with internal thermal diodes, you'll have to resort to thermocouples. Just place a thermocouple probe on the IC and you can measure the case temperature. From the datasheet you can compute the junction temperature, if you want that is.
If you are looking for something a bit more sophisticated then there are several temperature sensors available. Check out national's website **broken link removed**