Pick one up before you buy it, if you possibly can. Weight tells you almost everything. A small ornament that feels too light for its size is usually hollow resin dressed up to look like something sturdier, and it will chip the first time it gets knocked off a shelf by a cat, a kid, or an overenthusiastic dust rag.
The difference between a good one and a disappointing one shows up in the details nobody photographs. Run a finger along any seam or joint, if there's a visible line where two halves were glued together and it catches on your skin, that's a sign of cheap molding. Good ceramic or carved wood pieces have a seamless finish, or if they're jointed on purpose, like a hinged box or a figure with a separate base, the fit is snug with no wobble. Check the bottom too, a felt pad glued on crooked tells you something about how much care went into the rest of it, and an unglazed or rough base on a ceramic piece usually means it was finished properly rather than rushed through a kiln. Glass ornaments should feel cool and a little heavy for their size, with no air bubbles distorting the surface unless that's clearly part of the design. I've bought the lightweight version more than once, drawn in by a nice shape on a shelf in good lighting, and regretted it within a season. As for where they sit, small ornaments do their best work in groups of odd numbers, three or five, on a mantel, a windowsill, or clustered on a side table with something taller behind them for scale, a lamp or a framed print. Scattering single ones around a room makes a house look like a yard sale in progress. Group them by material or color instead, mixing wood with wood, glass with glass, and the whole thing reads intentional even in a rented apartment with walls you can't paint.
Wear and aging depends almost entirely on the material. Painted finishes dull and sometimes flake at the edges after a couple of years of handling, especially if they get picked up and put down a lot, which small ornaments always do. Ceramic and glass hold their color indefinitely but chip at the first drop, so keep anything fragile off low shelves if there's foot traffic or small hands nearby. Dust with a soft, dry cloth rather than anything damp on painted or gilded pieces, since moisture lifts paint faster than you'd expect. Wood ones benefit from an occasional wipe with a dry cloth and nothing else, no oils, no polish, just leave them be.
Measure the shelf or ledge first, and leave at least an inch of breathing room on either side of each piece so nothing looks crowded or about to tip.