That's a great question, and not so simple to answer.
Even mass murderers and other evil people have a certain ability to fool at least some of the people, some of the time, that they're 'nice'. If nasty people were instantly recognisable at fifty paces, it would make life a whole lot easier, calmer and plainer sailing.
But they're not.
Part of the problem is that most of us have been programmed from birth to push down our intuitive gut-reactions that something is 'not right' about certain people, in the name of good manners.
Yes, OK, your lecherous neighbour makes your skin crawl, but when he leers over the fence and asks you how you're doing, you feel socially obliged to ignore you're inner dislike and urge to tell him to get stuffed, and start making small talk instead.
Ditto, when the nosy, tactless acquaintance starts milking you for very personal details about the latest drama or trauma in your life: so many of us feel obliged to answer, instead of telling them to mind their own business. Why? That would be rude, of course!
Times these interactions by a few hundred, or a few thousand, and the end result is that you get cut off from your true feelings about people. You've got so used to putting 'good manners' ahead of your gut reactions, you could invite Jack-the-Ripper for supper and not even realize something's 'off' about your guest until he starts sharpening his dagger at the table…
So how can you get more in touch with your innate intuition, which is always trying to give you clues about who's 'nice' (at least, for you) and who isn't? The following tool, called a 'mood map' can help you.
The instructions are simple:
1) Before a social interaction or phone call, rate your mood out of 10, 1 being really bad, and 10 being the best.
2) After your social interaction or phone call, rate your mood again.
If it went up - engaging with that person filled you up, boosted your mood, and made you feel good. There's a strong chance the other person was 'nice'.
If it stayed the same - it was probably a very superficial interaction, and needs further exploration.
If it went down - engaging with that person deflated you somewhat, brought you down, and dissipated your energy. There's a strong possibility that the other person was 'not nice'.
If you pay attention to your mood maps, you'll start to get some profound insights into your relationships that you might otherwise not be aware of. If someone is consistently making your mood drop, consider ways to minimise the time you spend with them.
I know it's not 'good manners', but doing so could save you huge amounts of trouble, heartache and stress.