Like if you're defending or if you want to catch an enemy lurking, how do you hold an angle so that you can kill enemies easier? ( I also have trouble with off angles as well )

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...