The first thing I do with a handle or knob is turn it over and check the back, because that's where the cheap ones give themselves away. A thin shell of metal with a hollow ring to it, a screw post that wobbles before you've even mounted it. Pick it up and it should feel like it has some weight behind it, not like it came out of a cereal box.
On color and finish, I've learned to trust the ones that don't try too hard. Warm brass, matte black, plain brushed nickel, aged bronze, these all age well and sit quietly against most cabinet colors without fighting the room. Bright polished chrome looks sharp on day one and shows every fingerprint and water spot by week two, which matters a lot more on a kitchen cabinet near the sink than on a bedroom dresser nobody touches with wet hands. I went through a phase with a set of shiny gold knobs that looked great in the store lighting and then looked like a cruise ship lobby under my actual kitchen bulbs, so take a sample home before committing to a whole run of them. Where they clash is usually obvious once you see it: ornate, fussy pulls on flat modern slab cabinets look bolted on and wrong, and sleek bar handles on an older, detailed farmhouse style piece can look like they wandered in from a different house entirely. Mixed metals can work, but mixed metals that weren't chosen on purpose just look like you ran out of the first box halfway through, which, to be fair, is usually exactly what happened.
Sizing is where most of the damage gets done, long before anyone notices the finish. Measure the existing hole spacing on your drawers and cabinets before you buy anything, center to center, and write it down somewhere you won't lose it, because buying a gorgeous pull with the wrong hole spacing means either new holes, filled old holes, or a very awkward compromise you'll see every single day. For drawers, a pull roughly a third of the drawer's width looks balanced, shorter and it reads stingy, longer and it looks like it's trying to be a handrail. Knobs on tall cabinet doors want to sit at a comfortable hand height, not jammed in the corner because that's where the template said to drill. Over a year of actual use, the finishes above the sink test themselves. Food on fingers, dish soap, kids who treat drawer pulls like monkey bars. The decent ones wipe clean and barely age. The bad ones go dull and sticky looking within a season, no matter how good they looked under the store lights.
So before I buy, I ask myself: will this still look right in five years when the finish has had real life done to it, not just a photo taken of it?