The hardware is in good shape
Look for metal zipper with a hook-and-eye clasp, snap, or button at the top to keep it in place. The zipper should be hidden with a placket and should run up and down smoothly. Give it a test run to make sure it zips smoothly and doesn’t snag. For other types of hardware, like metal buttons and design features, give them a quick texture test. Does it feel light, or like it’s made of plastic? Quality hardware will have some weight to it.
It isn’t totally sheer
No matter if a top is meant for a girl’s night out or an afternoon at the office, sheerness is a fairly good indicator it wasn’t built to last. Lightweight and expensive materials like silk and cashmere aren’t exempt. Before leaving the dressing room, put your hand under the fabric and hold it near the light. If you can see your hand’s outline, the garment will likely be sheer in most lighting.