How to Measure FM Transmitter Power Built with Transistors Over 2km Distance

how can i measure the power of an fm transmitter with a distance of 2km the transmiter was build with transistors

Depends on the terrain, frequency, sensitivity of the receiver, noise immunity, type of carrier, quality of information transfer, presence of interfering signals, and in some cases, time of day (did I forget anything?).

Will this be over land, underground, through a tunnel, over/under water, in outer space, etc.?

You would need to know the transmitter antenna gain and its radiation pattern. The field strength meter would need to be of a calibrated type. There would then need to be a (complex I'm sure) mathematical formula to get an approximate power reading. Thus it would be easier to contact the owner of the equipment and ask about it. I am not trying to be funny here but what you ask is a difficult thing. It may be worth telling us why you want to know, and this may also be helpful to us and this will enable us to help you.