I am currently stuck in silver and I see so many posts about playing against smurfs and how it sucks but how can you actually tell it's a smurf?

How do you know the difference between "Me and my team just played bad that game" and "We got destroyed by a smurf"?

What if they are just in the zone during that game? because in some games I popped off but that doesn't make me a smurf.

Also how rare are they? is it pretty rare so every once in a while you get destroyed by a smurf or is it very common? meaning one of the reasons holding me back is smurfs?