Is X halal? (reference question)

Islam.SE often gets questions of the kind "is X halal?", where "X" may be quite specific and the permissiveness of which is very clear. Examples include [can I bathe with my wife](https://islam.stackexchange.com/questions/36836/a-question-ask-by-lads-about-couple) , [can I marry two women at the same time](https://islam.stackexchange.com/questions/36529/is-it-permissible-for-a-muslim-man-to-marry-two-women-at-once) , [can a wife accompany her husband in public](https://islam.stackexchange.com/questions/36790/can-a-wife-accompany-her-husband-in-public) , [is eating mushrooms halal](https://islam.stackexchange.com/questions/9449/is-eating-mushrooms-allowed-in-islam) . Basically the principle of "everything is halal until proven otherwise", together with the pragmatic rule of thumb "do your best, and relax" should solve almost all of these issues. However, a slightly more involved answer might be more satisfying to have. Therefore, these questions: - What is the default ruling for any action? - When is a Muslim required to check whether an action might be impermissible? (*) - What effort is a Muslim expected to put into determining whether an action that falls under (*) is haram?