OK such a can of worms. To play around is less intense than to fool around. Foolishness is more serious/more base than simple playing around. To mess/screw/f*** around are in the correct order of seriousness, from least (mess around) to the strongest. These last 2 can also mean to sleep around, if that is what you are talking about. To 'muck' around is similar to 'to fool around'. To distinguish between them - read my comments, which is mild, which are serious. And the voice intonation will help you know if someone is serious or rude when they describe them. Or you can just learn the words, but you yourself can use polite words instead.