Paper dictionaries usually have something called a radical index, where you find the character by its main feature and the number of additional strokes. But the best solution is to use more modern technology - various online and mobile dictionaries have a feature that lets you draw characters with your mouse, touchscreen etc and looks them up for you. The best one for Chinese is nciku.com; another dictionary with that feature is mdbg.net, but that one needs you to get the correct stroke order, which you might not be able to figure out for an unknown character. Here's one I found for Japanese: http://kanji.sljfaq.org/kanji13/draw.html - this one also seems to need correct stroke order, so there's probably something better available if you google hard enough.