Many methods:
1. Check if he / she have suddenly offline in the conversation without ny ending words (byebye, 88 etc.) except if he/she s busy.
2. If he/she have not been online for a long time (except if he/she is busy or do not use MSN anymore as he/she told you)
3. (Imagine the situation) If you have a friend A,B,C,D and you are E. If you five guys got to discuss something but D blocked you while A start the topic, D will be set as 'offline'. (But in fact, D is online but he/she blocked you.)