I've been told on more than one occasion that you can tell planets apart from stars in the night sky because stars twinkle and planets have a more steady light. 

I've never seen a sky full of twinkling stars. They shine with a constant light, not fluctuating visibly. 

What's weird, though, is that sometimes somebody will point out a planet, and explain that they know it's a planet because blah-blah-blah and it isn't twinkling. And I'll look up at where they're pointing, and wonder how it's possible that I see the only dot of light in my field of view that isn't supposed to change as the only one that is fluctuating.