By "Thermal Heatsink", do you mean the Thermal Paste that came pre applied to the CPU Cooler heatsink? If so, as long as the Thermal Paste still covers the entire surface of the heatsink without any gaps, in my opinion, it should be okay. If not, remove the thermal Paste using alcohol and apply new Thermal Paste if you have any doubts.
Install the CPU Cooler on the CPU and power up. If your CPU stays cool during idle than the thermal paste is working normally. But if the CPU is running hot, then you may have to remove the CPU Cooler and apply Thermal Paste again.