Dan is correct.
Using a step scale, such as a Macbeth Color Checker, you can expose correctly, and then figure out the threshold speed just by looking at the curve from this. Then, assuming ISO 400 from the straight line, you can guesstimate the maximum speed by the method Dan has described. Just look at the first detectable image and then measure the number of stops that this takes place at, vs the mid scale.
This is the threshold speed and is the max that can be obtained from that film by any means.