Yeah, I sometimes stand at a bus stop and practice some of my flourishes in an absent minded kind of way, I'm not really looking at my hands or taking notes, just going through the motions. When people see this, they usually immediately start to ask questions like "Wow, are you a magician? Can you show me some tricks?".
If you want to approach random people on the street, just look for someone who doesn't look like they're in a hurry, or busy... Look for someone who is having a bit of fun with their friends. You better be dressed in a decent kind of way and not look like you haven't had sleep for days, and then all you need to do is approach them and ask simply "Can I show you something with a deck of cards?", and if they say no, just nod, say "Okay, thank you" and go away.