This is lifecycle assessment - the practice of summing up all the conceivable positive and negative impacts of alternative techniques and technologies. It's not trivial to try to imagine and keep track of every possible ramification of a product - and you can quickly get bogged down in the weeds - but it's essential if we're gonna decrease our collective footprint.
On Point recently discussed this problem. In particular, they discussed Good Guide, a website and iPhone app that attempts to quantify the environmental impacts of various products you might buy.** Most people quickly get overwhelmed into inaction by long lists of the negative implications of their actions - so it's important to somehow distill the information down into something actionable. The purpose of this website is not to perfect your Green lifestyle, but to allow you to choose the best among limited options.
At one point an agricultural science professor called in to ask who defines "sustainability" on this website - especially as she considers industrial ag (with genetic engineering) to be more sustainable on the whole than organic ag. It's a good question, to which they had a pretty good answer.
They don't just assign a single number to indicate the sustainability of a given product, but rank them separately based on different concerns - e.g. if you are concerned about bringing toxins into your home, but you don't care much about global warming, you can just focus on the former.
I think we probably have a long way to go before we really understand the environmental, social and economic impacts of our product chains, but we've got to start somewhere. An educated guess is better than nothing, and if millions of us take that small step, it could really add up.
* Good thing I get all my books from the library!
** I'm not 100% sold on the way this website is set up, but it's still a great start