You might want to check AOAC method No. 997.08 (AOAC, 2005), but basically you extract inulin with hot water, and hydrolyse using inulinase (e.g. Sigma–Aldrich®, USA, I2017 inulinase from Aspergillus niger, CAS Number 9025-67-6), followed by a derivatization step of the  hydrolysed fractions were derivatised into volatile oxime-trimethylsilyl derivatives. Analyzing each individual sugar, both before and after enzyme hydrolysis by high temperature gas chromatography will give you a calculated total inulin concentration. For more details see: Chanantita Chaito et al., Food Chem,  doi:10.1016/j.foodchem.2014.10.140 and citations therein.

Look at this paper which has been cited many times

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