If you know the percentage of cellulose and hemicellulose in a lignocellulosic biomass then calculate the concentration of the sugars from the concentration of the corresponding monomeric sugars, using an anhydro correction of 0.88 (or 132/150) for C-5 sugars (xylose and arabinose) and a correction of 0.90 (or162/180) for C-6 sugars (glucose, galactose, and mannose).

Reference: Determination of Structural Carbohydrates and Lignin in Biomass- Laboratory Analytical Procedure (LAP): NREL method.

It depends on the biomass you are using. Composition varies for different biomass. Once you know your cellulose and hemicellulose composition you could calculate pentose and hexose sugars percentage as mentioned by Diwakar (but it will be just a theoretical value). 

The percentage of hexoses and pentoses in lignocellulosic biomass depends on the percentage of cellulose and hemicellulose respectively. Hemicellulose, a heteropolymer, contains 5 monosaccharides (pentoses and hexoses). Cellulose, a homopolymer, contains glucose (hexose). 

For composition of biomass, as mentioned by Arun, refer the below link:

www.katzen.com/ethanol101/Lignocellulosic%20Biomass.pdf