The first thing I check on a heart necklace is the join where the heart meets the chain, because that's where the cheap ones give themselves away. If it's a single soldered point with a visible blob of metal, the heart will swing forward and sit face down within a week. A good one has a proper jump ring or a bail that lets the pendant hang flat and centred, so it actually faces out when you look down. That small bit of engineering is the difference between a necklace you forget you're wearing and one you're forever flipping the right way round on the bus.
Length matters more than people admit before they buy. A heart pendant on a short chain, sitting just below the collarbone, reads as deliberate and a bit romantic. Drop it to the chest and it can start looking like something from a school disco, especially if the heart itself is large or glossy. If you're buying for layering, go shorter rather than longer, since a heart on a long chain tends to disappear into other pieces or get lost under a jumper, and you end up paying for a detail nobody sees. Watch the clasp too. Lobster clasps are fiddly but secure, spring rings are easier to open and more likely to let go without you noticing, usually somewhere unhelpful like a train platform.
On quality, weight tells you most of what you need to know. A hollow heart pendant that feels weirdly light will also dent if it knocks against a desk or a seatbelt buckle, and once it's dented it never sits right again. Solid castings cost a bit more but hold their shape for years. Plating wears fastest at the edges of the heart and along the chain links that rub against skin or clothing, so look at those spots rather than the flat centre, which always looks fine in the shop. Gold tone suits most skin colours and most outfits without trying, silver tone reads a touch colder and sharper, and rose gold is lovely in autumn and winter but can look slightly try hard with summer tans and bright colours. For everyday wear, plain gold or silver, no stone, no engraving, lasts longest in your actual rotation because it goes with everything without a second thought.
The mistake is buying the necklace with the biggest, shiniest heart because it photographs well on a stand, then finding it's too heavy, too loud, and catches on every scarf you own.