My method involves loading the CPU on a passive cooled heatsink at low clocks and volts, and using an infrared thermometer to check the base of the CPU (either back of the mobo, or just the base of the CPU heatsink) and comparing the temps.
An IR thermometer reading of the back of the mobo or base of the heatsink doesn't tell you too much about the actual core temperature at the hottest spot on the core. Even when I was testing without a heatsink installed, I still wasn't measuring the actual core temperature but at least I was getting closer.
When testing my E8400 I removed the heatsink, dropped the voltage down to about 1.10 volts and I dropped the MHz down to about 2000 MHz. At idle, 1600 MHz or 2000 MHz didn't make any significant difference. Based on rge's testing with a calibrated thermocouple and IR testing we came to the conclusion that the peak core temperature has to be approximately 5C higher than the measured IHS temperature. If you think about how heat dissipates, that seems reasonable to me.
I also used a hand held fan to stabilize the CPU temperature. An E8400 had no problems at all running without a heatsink as long as it had some air flowing over the IHS. My old 7000 rpm AMD Athlon heatsink fan finally came in handy for something. When using a TJMax of 95C, the reported core temperature and the measured IHS temperature were pretty much the same. Knowing that the hottest spot on the core has to be higher than the surface temperature of the IHS based on heat dissipation, it only made sense that TJMax must be higher so 100C sounds reasonable to me. Intel released that number at their first IDF conference and have never changed it.
I hope you can do some more testing mussels but I also hope you pull the heatsink off. As long as you have a fan handy, the CPU will be fine. Trying to keep a Q6600 B2 under control using this method isn't nearly as easy.
Pretty much impossible actually.
The base of the heatsink or back of the motherboard is going to be significantly cooler than the hottest spot on the core.
Dr. Blue: Core 2 and core 3 look stuck to me. My best guess is that core 0 is reasonably accurate as is with a default TJMax=100C and core 2 and core 3 are likely closer to TJMax = 103C. They both get stuck so are useless at idle. TJMax on Core 1 looks to be significantly higher. Maybe 105C for that one. It also has slope error so if you correct for TJMax then you'd need to run a calibration formula and correct for problems at idle. I wouldn't bother. Just keep in mind that it has a few more issues than the other 3.