You are now rummaging around in an IMHO utterly complicated area of the German language, or perhaps it is more related to German culture. The language aspect itself is not particularly complicated. I have been living as a foreigner for almost 25 years now in Germany and am still often enough not sure wether to use the polite or informal form when talking to people. Native speakers also have the same problem.
Many native speakers tell me to always use the polite form if I am not sure, but at least I feel awkward if politely addressed in a for me obviously informal setting, and I also know natives sharing the same opinion. Using the polite form in an inappropriate situation may very well signal snobbiness or a request to keep social distance, which is neither wanted nor intended. Only last week, a new waiter at my favourite pub actually said 'Sie' to me and honestly, my first gut feeling was somewhere between 'did I do something wrong, or am I already that old?'.
Quite often, this mess leads me and others to not directly address people, but somehow rephrase the sentence to avoid talking directly to someone, e.g. instead of 'could you (du or Sie?) please give me ...' you could express the same with 'I'd like to have ...' without having to choose between du or Sie. Even Janka's suggestion to 'never ever' use the polite form when speaking to family members is not without exception. It is not common, but I know Germans using the polite form when talking to their parents in law, and I don't have the impression that it is to convey a subtle 'I don't really like you'.
To make an educated guess, you must at least consider geographical, social and cultural aspects of the situation you are in.
Roughly speaking, Sie is more common the further north you are in the German speaking area. In some districts of Austria, people are commonly not using Sie at all, but du in all situations. I have even read Austrians describing the formal address as a completely inappropriate 'prostration to a superiority' and in any situation out of place. In Munich, I have occasionally been addressed with 'du' both in grocery stores and by the police. Considering that saying 'du' to the police elsewhere in Germany could cost you a hefty fine, we are even moving in and out of indictability by choosing the right words.
In most social situations, I would say that 'du' is always appropriate, perhaps unless you are together with clearly much older people or together with professional colleagues, which you in a non-social setting would address with 'Sie'. It has during the 25 years I have been living here becoming much more common to use 'du' also in professional situations, both at work and when being addressed as a customer e.g. in a shop or on a web page. IKEA was one of the first companies, which consequently addressed all their customers with 'du'. For a few other examples: Amazon is using 'Sie', but the customer's first name and eBay is not consequent with a mix of both 'Sie' and 'du'.
Then you have different cultural settings, in which you simply have to learn how to behave. In internet discussion forums, it is mostly considered rude to use 'Sie'. Radio amateurs also come to my mind. They are also always using 'du'. It is in many companies and also other organizations policy to use 'du' in any situation.
So, good luck! Whatever you do, you are at some point bound to do something wrong, but don't let that prevent you from speaking German ;-)