The thing nobody tells you about a knitted polo is that the collar can go soft and floppy within a few washes if the knit is too loose to begin with, and once that happens no amount of ironing brings it back. It looks sharp on the hanger and sad by October. And yet a good one is worth the hunt, because it sits somewhere between a proper shirt and a jumper, smarter than a t-shirt without ever feeling like you've made an effort, which is exactly the point on days when you can't decide what you are.
In the shop, get your fingers into the collar and the placket first. A collar with a bit of structure, some actual rib rather than a floppy single layer, will hold its shape after being stuffed in a bag or balled up in a gym kit. Check the buttons too, proper horn or resin ones sit flatter and don't crack the way cheap plastic does after a few turns in the washing machine, and a shirt with only two buttons instead of three usually means someone cut a corner on the placket to save fabric. Hold it up to the light and check the knit isn't so open that you need a vest under everything, unless that's genuinely your plan, in which case fine, but know it going in. For wearing it, a knitted polo does a lot of quiet work with a pair of straight leg jeans and loafers, or tucked into a midi skirt for something that reads a bit more put together without trying. It also sits nicely under a blazer for the kind of day that starts as an errand and ends as a meeting, which happens more often than it should. What separates the ones that last from the ones that end up at the back of a drawer is mostly the yarn. A cotton with a bit of weight to it keeps its shape wash after wash, where a thin cotton blend goes baggy at the hem and cuffs within a season, especially if you're someone who rolls sleeves up without thinking. Check the ribbing at the cuff and hem does its job of holding in, not just lying there decoratively, because that's the bit that tells you whether it was made to be worn or made to be photographed on a rail.
Washed properly and not tumble dried into oblivion, a decent one keeps its shape for years, the kind of thing you reach for without having to think about whether it still looks right, which, for a knitted anything, is no small feat.
Would I still want this if the collar had already gone soft by the time I got it home?