Knitted polo shirts look neat as anything on the rail, close knit, proper collar, that little row of buttons sitting flat, and then you get it on and realise the knit has far more give than a woven shirt ever would. Buy your usual size out of habit and by the end of the day it can be sitting loose across the body and baggy at the hem, especially if the yarn has any stretch in it at all. It is still worth the bother, because nothing else gives you that slightly dressed up, slightly off duty look without actually trying.
In the shop, go straight to the collar and the cuffs first. Press the ribbing between your fingers, it should spring back rather than stay flattened, because that tells you how it will behave after a few washes rather than on day one. Check the depth of the button placket too, a short one gapes the moment you reach for something on a high shelf, and have a look at the buttons themselves, proper ones sit flat and are stitched through more than twice. Side vents matter more than they sound like they should, a polo with a bit of a slit at the hem moves with you when you sit down, one without just rides up at the back. For wearing it on, a knitted polo does a lot of quiet good work over straight leg trousers or jeans with a bit of structure, tucked in loosely rather than stuffed in, and it sits well under a blazer or a lightweight jacket when the weather cannot decide what it wants to do. With a midi skirt it reads smarter than its price tag, which is a useful trick to have.
The difference between one that lasts and one that does not comes down to the fibre more than anything else. A tighter gauge knit in cotton or a cotton and linen mix keeps its shape through a season of actual wear, while a loose, airy knit looks lovely folded on a shelf and goes out of shape the first time it is balled up in a bag after being taken off on a warm afternoon. Look at the shoulder seams, they should lie flat rather than roll, and check the inside of the collar for a bit of reinforcement where it meets the placket, because that is the part that curls first on the cheaper ones. Elbows are the other tell, a thin knit there will show wear within a few months, a denser one will not.
Would this still look sharp after a full day, a wash, and being stuffed in a bag on the way home?