Hmmm out of the ordinary... what you mean by that... examples please? thats too vague..
Hmm like sazzy said... They act nice over something in return or something of their interest... Well like her examples ^^ which is very true...thats how they are like... it's most likely very hard to know they are "acting out of ordinary" why would you watch closely over someone who you "think" is nice... ?
Well...first of all... i think you must evaluate a friend before being close to him/her... study their backgrounds a bit then slowly look at whats he/she done in the past...if you find something fishy about him/her..