Sustainability implies that products offered by a company are eco-friendly, adhere to social or economic issues, or follow ethical principles in their production. So, first, define to which category a brand that you are doing research about belongs. Then, go to the company’s website, learn about its history and mission, and read through all the available information. Bigger companies mostly have certifications, e.g. Fairtrade or Global Organic Textile Standard. Companies may also provide links to the code of conduct or code of ethics on their websites.