There's been a number of posts and threads about this over the years, and there's a document on http://ebikes.ca for it too, IIRC.
https://endless-sphere.com/forums/search.php?keywords=shunt+value&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search
https://endless-sphere.com/forums/search.php?keywords=shunt+resistance&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search
But basically:
If you have a way to make a known constant current thru a resistance, that you can then measure teh voltage across, you can calculate it's resistance.
Ohms = Volts / Amps
(Rock sees the Eagle over the Indian; R = E / I)
Alternately you can apply a known voltage across the resistance, and measure the current thru it, but generally it's easier to more precisely measure volts than current with most of the devices people have laying around.
You can also figure out the resistance by successive approximations, if you're using the shunt as the source for a measuring device like the Cycle Analyst. If you know what current it *should* be measuring, by comparing with a meter in series with the shunt that you know is correct, then you can set the CA to some arbitrary value, and if it's too high, then lower teh shunt value in setup, if it's too low, raise it. Change first by half the value or twice the value, and as you get closer, use half the change you made before, untl you get it to read the same as the second meter.