How to detect the presence and magnitude of multipath error on a fixed carrier freq.?

Anyone have any good ideas on how to detect the presence and magnitude of multipath error on a fixed carrier frequency. In other words, if I transmit from point A to point B, and I measure the phase shift (or time delay), if there is an echo coming via a multipath source that is adding a phase shift error, how might I detect how much phase error the received composite signal has? I need a narrowbandwidth solution.

Hello biff44

You need to send a known signal at point A. The repetition rate of this signal has to be larger than the greatest multipath reception that you except at point B. This signal could be a sine wave or (better) a PN-sequence.
At point B you have a correlator for the known signal. The correlation of the reception signal gives you at least one peak, this is the (unreflected) main-carrier. All other peaks are reflected multipath signals. Their position gives the timedelay (phase) and the amplitude is their reception strength.

This method is also used by telephone modems to set their equalizer-filters adaptively at the beginning of a communication.