They have to know that their boss isn't going to slam them into smithereens for asking the caller another question. Rushing is another sign. Name dropping is a biggie. They can look up the name of a senior manager in your annual report. They won't use his name, they'll use his wife's name.
Intimidation is another tell-tale sign.
We like to have our egos stroked. Requesting forbidden information is also a tip-off. If someone's says they need your password, they're lying, don't give it to them. Educate, educate, educate. That's probably the only way.