The internal sensor measures the temperature of the die (silicon). An external sensor can never show you an accurate measurement since the die is inside a plastic package and the best the sensor can do is to measure the temperature of the package (outside of the IC), which is cooler than the die.
Motherboard Monitor is an example of a software utility (free) that can monitor and show the (internal) temperature of your CPU. It will show it on the taskbar.
Control of the CPU fan RPM, based on the CPU temperature, may be available to you. That depends on your motherboard.
I have never used an external sensor. It may be best to put it on the other side of the motherboard right under the CPU. If you put it on top, to be close to the CPU, you will have to place it under the heatsink, between the heatsink and the motherboard. That is not a good idea since it may hold up the heatsink and not allow it to make a good contact with the CPU.
Wherever you place it, you should make sure that it does not interfere with the interface between the CPU and its heat sink.