If you could magically identify smurfs, you could just put them at the level they should be or ban them without putting them in a match

That’s what league does. If it detects early on that you’re probably a smurf, you get tossed into smurf queue where you’ll face other experienced players, and it’s pretty reliable. Both times I’ve made new accounts, I’ve almost immediately gotten placed with experienced players, usually before the account graduates beyond bot games