You could assay for the generation of reducing sugar ends, e.g. with the dinitrosalycilate or the Nelson-Somogyi assay. Alternatively, you could use a chromogenic assay using p-nitrophenyl xylobioside, commercially available (but expensive)

As Prof Beguin mentioned, you can estimate reducing sugars released in an enzyme assay involving xylan (from beechwood or oat spelt) as substrate. You can follow methods described in our paper (link below)

https://www.researchgate.net/publication/289376884_Improvement_of_wheat_straw_hydrolysis_by_Cellulolytic_blends_of_two_Penicillium_spp

Article Improvement of wheat straw hydrolysis by Cellulolytic blends...