How to detect thermal throttling?

In order to find out whether your CPU is actually throttling, you should be able to use Intel's Extreme Tuning Utility (XTU). I run it on my Ivy Bridge (3rd Gen) systems, but it should work the same on your Sandy Bridge (2nd Gen) CPU.

The Extreme Tuning Utility can directly indicate whether the CPU is throttling or not (see screenshot), and it is also a very convenient tool for observing other information such as CPU utilization, temperatures and frequencies.

Prime95 can show you the exact CPU speed, but you don't need any tools to confirm that. It is obvious that your CPU is overheating. Stop the server, make sure the fans are working and apply new thermal paste on the dies.

The CPU temperature at full load should not exceed 60°C. You are significantly over the limit.