I wonder how to find out the radical of a characters, I understand that for example 你 has the radical 人 (means the radical is in the front) but there are others where radicals are on the top or even in some other place of the character.  For example 问 where the "door" is surrounding the mouth, or 名 where I just learned the radical is mouth !

Is there any rule on how to find that out? I think the identification of the radicals is important for looking up words in the dictionary if I do not know the pronunciation.