3rd party certifications should be more objective/fair than what the company will say about itself. There are so many now, Made Safe, GOTS, FSC, RWS, C2C. Other than that, does the company repair their products, have a long warranty, sell their products used, etc? Here are some websites:

https://www.bcorporation.net/en-us/find-a-b-corp/ https://earthhero.com (like Amazon, but only sustainable products) https://www.treehugger.com (Lists of sustainable products)