Decades ago, a friend of mine at the time (female) met a guy. She was really beautiful and a genuinely nice person, so lots of guys would ask her out. She told me about a date with this one guy, and he didn't call her again. Even though she hid some of the details, I was able to ascertain the general contours of what happened and was able to figure out his agenda. How? Because of something he said to her.
She told me that he picked her up and they went out for diner, but when he dropped her off at home he became concerned about parking, wondering if a garbage truck might pin him in the next morning. At the time, she thought nothing of that. But she explained to me that she hadn't planned for him to stay overnight.
But what he said was a pretty BIG siren. It was obvious he already intended to stay the night, otherwise why would parking the next morning matter? My guess is that if she had known his agenda she would have viewed him differently and maybe been more wary.
End of story? He never called her again. She was mystified as to why. I thought: well of course he didn't call--got what he wanted!
In response to your question, wouldn't say that there are telling signs, it's more about the content of what people say as the story above illustrates.
A few other loose thoughts about understanding people:
1) It's dangerous to assume that other people think as you do. In fact, people come from a range of contexts and backgrounds which are not obvious when you meet them. They are perfectly happy to smile and nod with you while completely disagreeing.
2) People are heavily self-interested. Unless it's your mother (and sometimes even then), expect people to look out for themselves first. Someone making a show of sacrificing for you? Hmm.
3) People enjoy feeling superior to others, even in very minor things. Shrewd people know this and use that information to their advantage. (Not advising this, just saying what people do).
4) Chinese proverb: the dog that can bite doesn't bark. When people have the capability to get you, they don't have to talk about it--they do it. The big talkers generally don't have the ability (though there is the occasional outlier).
You may want to look into some books on this topic. A useful one is 48 Laws of Power by Robert Greene. Not saying that you should follow his advice, but it is useful in understanding games that people play at work.
Book link (not affiliate) - https://amzn.to/2dzBqXD
Best of luck.
Malcolm X and the Nation of Islam met with the KKK in 1961 despite mutual hostility that would never be ameliorated . In 2016, your entire career can be ruined simply for holding the "wrong" opinion. Boy how things have changed.
"Even in such technical lines as engineering, about 15% of one's [financial] success is due one's technical knowledge and about 85% is due to skill in human engineering, to personality and the ability to lead people."
In this sense a few books about mastering that 85%: