Conventionally, soluble dietary fiber (SDF) is extracted from plant cell wall with hot dilute acidified water. Besides acid extraction, various other methods for extraction of SDF have already been investigated in order to achieve the desired quality and quantity characteristics of extracted SDF. These methods include enzymatic extraction, extrusion extraction, microwave-assisted extraction, and ultrasound-assisted extraction. Subcritical water, was also proved to be effective for extraction of SDF from citrus peel and apple pomace. Recently, we have prepared pectin (major component of SDF) from soy hulls using phosphate-assisted subcritical water combined with ultrasonic treatment (Liu, C., et al. (2016). Preparation and characterisation of surface‐active pectin from soya hulls by phosphate-assisted subcritical water combined with ultrasonic treatment. International Journal of Food Science & Technology, 51(1), 61-68.). This work may be for your reference for extracting SDF from soy hulls.

I will check that paper, thanks a lot for your help!

The standard AOAC method 991.43 should support your goal.  The method can report total dietary fiber (IDF and SDF) or the individual components.  I have no doubt we have done soy hulls in our lab.  There will be significant IDF in the fraction.