I have no clue how to tell if its mounted properly.
Usually the "wiggle test" is enough, with the PC turned off, see if there's any play from any angle. However, the only way to be 100% sure is to unmount it, clean the old paste out of both the heatsink and the CPU, drop a bit of paste on the CPU, mount it again.
Another question: is your graphics card really close to the CPU? Because I've had a situation where a graphics card was making both the CPU and the southbridge heat up pretty bad.