1. Digestion the sample to alter poly to simple

2. precipitate Protein

3. Fehlinc A+B titration brown color

or spectrophotometric  AOAOAC  

http://vddb.library.lt/fedora/get/LT-eLABa-0001:J.04~2005~ISSN_1392-1134.V_37.N_2.PG_34-43/DS.002.1.01.ARTIC

may be useful

https://www.researchgate.net/publication/230244105_Acid_hydrolysis_prior_to_automatic_analysis_for_starch. This may be of help.

Article Acid hydrolysis prior to automatic analysis for starch

You can use Total Starch Assay Kit from Megazyme. Procedures in PDF can be downloaded on google. 

Starch content of potatoes have already been determined and there is no need to waste your time in such experiments.