Bruce Lee has the most practical suggestion. While pressure and velocity may not correspond directly finding that a load you've worked up that produces 1100 fps with a 125 grain bullet from a 4 inch revolver is a pretty darned good hint that you are way over the recommended pressure for the 38 special. In fact that's a load result pretty typical for a mid range 9mm parabellum, so it wouldn't be too difficult to conclude pressure somewhere around 27-30,000 psi.