[StainlessSteelCynic]

Short answer - they don't. The whole idea of a rangefinder is to discover the range, it doesn't need to know the range before it fires.

Many early laser rangefinders fired a beam at the target and by measuring the flight time for the beam to hit the target and then be reflected back to the rangefinder's sensors, calculated the distance from that info. This is the Time Of Flight method, i.e. the amount of time a light beam will take to travel a certain distance (given that the speed of light is a known value).

Another method more commonly used these days is Multiple Frequency Phase Shift where a beam of known intensity/wavelength is measured upon it's return from the target so as to compare the difference in power modulation of the fired beam and the returned beam.
Or for a very crude explanation -- measuring the amount of energy/wavelength left in the beam when it returns compared to the amount of energy/wavelength used to fire it.

A third method is based on Interferometry where the laser beam is split into two distinct beams (that then share exactly the same wavelength/waveform) which are fired at the target but travel two slightly different paths to the target. Upon their return the two beams are recombined into a single beam before striking a detector. The difference between the two wavelengths, i.e. the interference, is noted and this is used to calculate the distance travelled.
Again, for a very crude explanation, an Interferometer laser rangefinder sort of combines the time of flight and phase shift principles in one, to calculate distance.