For the minions, the special ones give you the message "This minion still looks afraid of you. Come back in about 24 hours to bond with it more." instead of giving you the option to buy it right away. That's how you know (because Dara tells you in the text at the top that "There are some minions that I couldn’t possibly give up unless there was a strong bond first. I will let you know if this condition arises." which ... he does by telling you THIS MINION IS AFRAID). :)