How can we detect the source of the radio emission?
Every Government does things slightly different, but the way most organizations look for illegal wireless use is to use directional antennas. This is usually an array of antennas to cover a decently large frequency range and direction. They will then stick this array on top of a van and drive around. If an antenna shows a strong signal then they can know to travel in the direction that the "hot" antenna is pointed. They can then just play a game of Hot and Cold until they find where it is coming from.

Most likely if you are using a frequency that is allowed, but at slightly higher radiated powers, they probably wont notice. There is enough variations that can be caused by various objects in its path that it would be hard to tell if you were radiating too much or not.

If you are using a frequency that is not allowed, but keep the power low enough that it would just look like noise at the street then you will also most likely be fine.