Check the Google Shopping feed
Check the shop's Google Merchant Center product feed against Google's product data specification: id, title (150 characters at most), description, link, image_link, availability (in_stock, out_of_stock, preorder, backorder), price with its currency, condition, brand with GTIN or MPN (or identifier_exists), GTIN check digits, duplicate ids, links to another site, and price and availability matching the shop on a sample of products.