How to tell if CPU is actually overheating or is a false positive?

How does the CPU know it's temperature? I've been having trouble tuing on my computer as I keep getting "CPU fan error". I reapplied thermal paste and got a new heatsink/fan and still the same problem. Could it be a false positive? How can I tell? Any other causes of the error that don't actually have to do with the CPU heat, for example could the CPU not be correctly seated? The heatsink I'm using is stock Intel and I heard all 4 pins click in. Plus when I took it off the thermal paste had been smudged around so I knew it made contact, and I watched the fan spin.