A good statement earring has a bit of heft to it before it goes anywhere near your ear. Not heavy exactly, but present, the kind of weight that tells you the metal isn't hollow and the stones aren't stuck on with something that'll give up by October. I pick one up in a shop and give it a small shake. If it rattles or feels like it's held together by hope, it goes straight back on the stand.
Before you buy, have a proper look at the back of the thing, because that's where the cheap ones let you down. A flimsy butterfly fastening on a big drop earring is asking for trouble, it'll bend the first time you catch it on a scarf and then the earring sits crooked for the rest of the night. I look for a decent sized post and a back that actually grips, and I check where the weight falls once it's hanging, because an earring that's heavier at the bottom than the top will swing forward and knock against your jaw all evening, which is not what you want when you're trying to have a conversation and not look like you're nodding along to something nobody said. Plating wears thin at the edges first, usually where the metal meets your skin, so run a finger over any exposed bits and if they feel rough or you can see base metal underneath, that's your answer about how it'll look in a year. Mine live in a little dish on my chest of drawers rather than tangled in a jewellery box, because statement pieces catch on everything and a snapped chain link isn't something you can fix with superglue, trust me on that one, I've tried. A soft cloth after a night out, keeping them away from perfume and hairspray, and they'll keep their shine a lot longer than you'd expect from something that cost what it did.
Fit matters more than people assume with earrings this size. It's not just about your earlobe holding the weight, it's about where the earring actually sits against your neck and jaw, and whether it gets lost in your hair or swallowed by a big collar. Try them under the light you'll actually wear them in, not just the shop mirror, and move your head properly, because a shape that looks striking standing still can turn into something that pulls at your earlobe and tips sideways the second you turn to talk to someone.
The bit I love is catching sight of them in a shop window reflection on the walk somewhere, completely by accident, and thinking, oh, that's a good one.