Scripture. If someone can show me a scripture, or a scriptural principle, then we can consider and discuss and see if we come to an agreement. If they can't show me scripture or a scriptural principle, then someone's created a man-made rule.
As you pointed out in an earlier response, use Galatians 5 and the fruits of the spirit; and test your own motivations for those fruits (i.e., am I being truthful because of some thought that I'll be rewarded for it in some way, or simply because it's the right thing to do?). Look at the other portions of scripture mentioned (Eph 4 and 5, for example) and test your own motivations again (am I striving for unity because I expect to be rewarded for doing so, or because it's the right thing to do? And/or simply because I'm obeying scripture?).
The nit-noys we get wrapped around (smoking, movie theaters, etc.) are the things that we need to discuss whether there are scriptural principles that apply or not...whether there's a biblical principle that can actually be applied to them. For example, going to the theater in and of and by itself can be classified as harmless. If the theater is showing a movie which is identified as a comedy (and elicits laughter and "fun") about teen-agers attempting to have pre-marital sex, then what does scripture have to say about pre-marital sex? From there, what does scripture say about identifying pre-marital sex or attempting to have pre-marital sex as fun and deserving of laughter? From there, what does scripture say about those that support pre-marital sex (i.e., showing the film and accepting money to show the film that )? And so on.
--- Edited to add ---
As a further example, I myself am not persuaded that drinking is sinful. However, I read in Romans 14 that Paul says he won't eat or drink or anything else that causes another to stumble or be weakened; so I choose not to drink.