You can tell outdoor clothing companies that are committed to sustainability and serious in their efforts to reduce the environmental footprint they have. They produce a durable product, make use of recycled materials and will only use organic cotton for example. They will have a named person in charge of something like Corporate Social Responsibility or Sustainability and will have a mission statement which includes measurable targets to achieve. They will produce an annual report on their progress in achieving these targets.

Patagonia comes to mind as a pioneer in this area, as a market leader setting sustainability standards.