The first thing I check is the band, not the cups. A bralette lives or dies on whether that elastic has any real recovery in it, because the whole point of the thing is a gentler hold than a structured bra, and if the band has nothing to give, you've got all the limitation with none of the support. I run it between two fingers before anything else. If it snaps back sharp, that's promising. If it just sort of slumps, put it down.
The gap between a good one and a sad one is almost always in what you can't see from across the shop floor. Cheap bralettes use a single layer of lace with no lining at all, so you get show through in anything pale, and the elastic is often stitched flat rather than enclosed, which means it curls at the edges after about four washes and sits there like a rolled up blind. A well made one has a bit of weight to the fabric, proper flatlock or bound seams that don't dig, and hardware, where there is any, that's been set so it doesn't sit right on the spine or under the arm. Cheap rings and sliders go cloudy and sometimes leave a mark. Good ones stay neutral against skin for years.
Honesty requires admitting the limits too. A bralette does not do a job for anyone who needs real lift or containment, and no amount of pretty lace changes that physics. I've got friends who are a generous full cup and they find bralettes comfortable for about an hour before everything's drifted somewhere it shouldn't be. If you need your bra to actually hold things in a particular place for a working day, this isn't that garment. It's closer to a soft, pretty layer than to engineering, and it should be bought with that understood rather than discovered halfway through a meeting.
It also doesn't sit well under anything tailored or fitted through the chest. The lace edges and any bow or trim detail will print through a fine knit jumper or a fitted shirt in a way that looks accidental rather than intentional, and a seamed cup under something clingy shows every seam. Save it for looser knits, slip dresses, or days when it's doing a job entirely of its own, visible or not, rather than hiding under something structured.
Before I buy, I ask myself whether I'm choosing it because it's genuinely comfortable and well made, or because it was the prettiest one on the rail and I've talked myself out of noticing the band's already gone soft in the packet.