The way you can tell a lace bra is cheap isn't in the shop, it's about six weeks in, when the lace along the edge of the cup starts to fray into little hard bobbles that catch on your jumper, and the underwire starts making itself known through a thinning patch of fabric at the side. A good one wears in. A bad one wears out, and there's a real difference between those two things that no amount of careful washing fixes.
Lace doesn't play well with certain things, and it took me buying one in a slippery satin colour to learn it properly shows every seam of a tight jumper, so under anything clingy or pale you'll see the scalloped edge print through like a tattoo. It's also not a bra for anyone who needs real support doing real things, running for a bus or a long day on your feet, because the stretch lace that makes it pretty is rarely the stretch that holds weight, so the cup gives before the band does and everything migrates south by lunchtime. The difference between the one that lasts and the one that doesn't is mostly in what you can't see easily, the lining behind the lace, whether the underwire channel is properly bound or just glued, whether the hook and eye has two rows or three, and whether the lace itself is a proper raschel knit, with some give and bounce to it, rather than a flat guipure glued onto mesh, which looks identical on a hanger and falls apart within a season because it's got nowhere to stretch when you breathe. Cheap lace also scratches, properly scratches, under the arm and along the back band, in a way that good lace, which is finer and softer because it's actually made with more thread per inch rather than less, simply doesn't.
In the shop, I always check three things before anything else. I turn the cup inside out and look at the lining, because a nice sheer lace bra with a decent opaque lining underneath will actually hold its shape, whereas an all lace cup with nothing behind it is basically wearing a doily and hoping for the best. I run a finger along the underwire channel to feel for any rough edge, because that rough edge is the one that will be digging into your ribs by October. And I check the strap attachment at the back, because if it's a flimsy little loop of elastic rather than something properly reinforced, the whole bra will slowly tip forward on your shoulders no matter how many times you adjust it, and you'll spend the whole day hoicking straps up in a way that is its own small, quiet tragedy.