You can run a utility that will monitor whether your CPU is throttling from which you can determine what the safe temperature is for your CPU.
You could run a CPU stress test utility which might well push your system into throttling and you then monitor the CPU temp when that happened. Some utilities will record the maximum CPU temperature reached which will be the figure to use.
The temperature readings from older CPUs are less accurate so the values you see may not be accurate in absolute terms but it doesn’t matter as it gives you an offset to use.
E.g. if throttling occurs at an indicated 85C then you need to decide what you feel is a safe margin to use and then keep an eye for a while to make sure that your CPU’s maximum temp stays below that.
I can’t recommend which utilities to use especially in regard to monitoring throttling but if you wish to pursue this I’m sure myself and others can help out.