Look up the specs for the processor in your system, then chart the temps alongside per-core clocks. On Intel chips with a 1 to 4 core boost, no cores will rise

Look up the specs for the processor in your system, then chart the temps alongside per-core clocks. On Intel chips with a 1 to 4 core boost, no cores will rise above the base clock when thermally throttled, voltage may increase, and frequencies will be lower. You can run a CPU bound benchmark until temps rise and note when the performance begins to decrease. On desktop Intel processors, this usually happens between 70-80°C.