Many times, when reading Mandarin, I will happen across a character that I don't know. Usually, the radical along with some context will clue me into the meaning, but if I can't determine the radical, I'm stuck. Generally, I can spot many character's radicals, such as 氵, 辶 or 宀 . However, there are characters who have one or more "radical-like" components to them, like 烋 or 想, and I'm unsure how to tell which is the true radical. Is there any general approaches which will at least more better than guessing? Are their certain radicals that generally take precedence over others when they both occur in the same character?

EDIT: My examples aren't the best, as both of those characters have radicals that are easy to pick out. A general strategy for when they aren't so easy is really what I'm after.