The best way to assess the sustainability of a certain product or process is by evaluating it from cradle to grave, this is called a Life Cycle Analysis or Life Cycle Assessment (LCA). When performing such an analysis it is important to first exactly define the important variables and what exactly should be assessed (goal and scope). A LCA question for example not “is product A or B more sustainable?” but rather “what are the CO2 emissions during the complete life cycle of product A and B?”. The complete process should be analyzed, including for example the transportation of the materials, which makes an LCA often very complicated.
This video explains in a short and clear way how a LCA is performed, using an iphone as an example.