Calculating resistance and capacitance given the time constant
For passive filters there is a term known as ORDER i.e. first, second, third order etc.
It can be found by plotting Frequency Amplitude diagram/graph..including corner frequency and the slope of the amplitude..
for first order one has to have two measuremet of output input at different frequencies.
for higher order more measuremet.more complex..simulatanious equations, matrices etc.


However assuming it isto be  simple first order filter: meausre input and output DC resistance..keep the probe firmly connected to output till resistance value stablises.
corner frequency= approx. (2*PifC)*R=1 and find out C.
vtingole