To do that you need to set relevant criteria. The easiest way of doing that is to include criteria from a sustainability certification, like TCO Certified, and ask for a valid certificate to verify compliance with the social and environmental sustainability criteria in the certification.
When specifying an ecolabel or certification by name, you may be required to accept equivalent proof with its underlying criteria. For example, it is currently mandatory for all public purchasers in the EU to accept every valid type of proof of compliance with the criteria you set. Evaluating the validity of the proof you receive from your vendor is therefore a central issue for procurement if they are really to drive sustainability. If not done properly, you may purchase products or services that are not actually meeting the sustainability performance you were looking for. To date, there is no official standard for determining the equivalence between different methods of proof submitted by bidders.