If you are meeting someone for the first time and you find you're attracted to them, flirt. Be obvious that you like them. Be positive, fun and happy. Don't play hard to get when you first meet someone.They will interpret it as you not liking them. If you are acting like you like them and not getting any response then you can safely say that the attraction isn't mutual.
If you need more time to get to know them to decide if you're into them or not it's still a good idea to be friendly with them. If you're rude or distant you could ruin your chance to get to know them better. Flirting is about making eye contact, flashing a smile, listening and asking questions. Show him that you want to get to know him better.
If you've made it obvious and you get no response you can simply move on to the next person you find attractive. You don't have to look back and worry that you we're being too reserved. You don't have to wonder if he's not sure if you like him. You made it obvious and now he can decide.
Men like to be liked. This does not mean that you should call him and profess your love for him. If he thinks you like him and he likes you back he will call you. If he doesn't have your number he will find it. Make it obvious that you find him attractive but allow him to pursue you. Men want to be in control of their own lives. They want to decide who to date and who not to.
He also wants to know that when he calls you'll say yes. If he's not sure he could shut down in fear and forget the whole thing. There is a fine line between being excited and being scared to death.
His actions will always speak louder than his words. If he's not in hot pursuit of you, "he's just not that into you."
If you've made it obvious that you like them and you get no phone call at least you can go to sleep at night knowing you did what you could. No regrets. Simply move ahead.