I'm always amused watching low-rated players accuse other people of being "smurfs." Part of being low-rated is that you're bad at the game (no offense; I was low-rated once and I was pretty bad). This means, among other things, that low-rated players are bad at being able to tell if someone is a "smurf."
There is basically no trick that a -200 can't do. They can make a ghetto gunship or they could ctrl-k a t2 mex in order to build a t3 mex on top of it. They can surround a mavor with pgens so it shoots faster and they can assist a shield to help it stay up under bombardment. They can dodge in circles to avoid notha bombs, etc. etc. etc. What separates a -200 from a 1500 primarily is not how many tricks they know. It's more about avoiding bad behaviors/mistakes, and about understanding what is important and prioritizing that.
If I want to know if someone is a smurf, I watch the replay from the accused person's point of view. I see what orders they queue up, how they balance mass/energy, and look for mistakes. I basically ignore clever tricks or amazing micro like hoverbombing. I'm looking to see how well they balance their economy and whether it even looks like they're trying.
As far as I can remember, every single time I've looked at a replay to see if someone is smurfing (not on behalf of the moderators/admins, just on my own) I see gameplay in line with the person's rating. If they win, it's because their opponents made a lot of mistakes. Not because they're a secret smurf.