The only way to make sure an account is a smurf is to study its profile. If you can spot repeated freeloses, then you are 100% sure the player is bypassing the MMR system, whether it’s willingly or not.
Depending on the type of smurf, there are other minor usable criteria (such as an amount of career games way to low relative to the MMR, or a winrate considerably higher than the usual over a significant amount of games, etc.). More on the dedicated thread if you’re interested.
Now there are plenty of elements which can make you suspect your opponent is smurfing before delving into his profile, but as you highlighted, none of them do constitute an element of certainty. 