I think this is a good question. From my experience, developing an understanding for sustainability is best done via "criticism, questioning, discussion, and voting" as Highly Irregular mentioned. The reason for this is that the definition of sustainability can vary greatly (depending on the scope with which you analyze something, or the domain you specialize in). For example, you can assess the environmental sustainability of a new 'eco-house' and that will give you different results whether you then decide to expand your analysis at a street or city level, by bringing in other metrics of analysis (social, economic, political).
Identifying the 'degree of sustainability of something' is best done by comparing with other similar things. That's what a life cycle assessment does. It is a tool that, given a particular process or product, it gives you its environmental and pollution impact from the source of the materials right till their disposal. The LCA returns a series of numbers, which, by themselves are generally quite meaningless since these numbers are calculated using Global averages (and as such, your product might have a different impact in your country compared to the global average). The value of an LCA comes when it is used to COMPARE similar products/processes and assess their relative benefits. So in response to your question, it is very difficult to identify what is sustainable without any comparison with other products/processes.
In terms of marketing ideas, Highly Irregular is right in saying that in many cases Marketing can be used to push the wrong ideas. The best example is the marketing of electric/hybrid vehicles. Often manufacturers claim that their cars are 0 CO2 emissions. Although this may be true on the car itself, these claims don't take into consideration where the electricity comes from in the first place. An electric vehicle is charged using the mains electricity. The mains electricity is generated using a mixture of power sources: coal, gas, renewables and nuclear (in countries with nuclear power). The CO2 emissions of your car strongly depend on when and where you charge your vehicle. If you charge it at peak demand hours, the likelihood is that a large portion of that electricity comes from coal, leading to tremendous CO2 impact of your electric vehicle. If you were to use such a vehicle in Australia for example, you would be releasing more CO2/km than using an internal combustion engine car. All of these issues are unknown to the general public, and marketers make sure to paint a nicer picture.
What has worked best for me in terms of discovering new and unique sustainable practices is to attend conferences, read papers and discuss ideas with many people who are either developing 'sustainable' products or carrying out research into the field. Often many interesting insights into sustainability can come about in this way. A curious fact: I met with a PhD who is researching methods to analyze the sustainability of homes. One of his 'assessment tools/evaluation tools' is a 'break things up' test. They suggest that when you design something, you'd want random people to try and break it in any way possible as those are the kind of unexpected things that happen during the lifetime of a product. His link with sustainability: "it isn't very sustainable for things to break unexpectedly, as that means that you'd have to replace them, thus increasing your environmental burden with your product".
I hope this answers your question in part!