Start with bar spacing, measured not guessed
Take a tape measure, not an eyeball. Check the gap between bars at several points, including corners and any place the bars have flexed, because spacing on a used cage is rarely uniform. The test that matters is your own bird's head: spacing must be narrow enough that the bird cannot push its head between the bars, since a head through a gap can mean a trapped neck or a panicked injury. Also check the gap between the tray, the grate, and the cage skirt, and the space around the door frame — birds investigate edges, and a beak-sized gap in the wrong place can catch toes or a lower beak.
Read the coating like a health warning
Run your fingers and eyes over the finish on every bar, especially around food-bowl doors, perch mounts, and the lower bars where droppings and scrubbing wear it hardest. Flaking, bubbling, chalking, or rust blooming through the coating are all rejection points, not cosmetic flaws. Birds climb with their beaks and will strip any loose edge, and you cannot verify what an older cage was coated with or whether a previous owner repainted it with something unsuitable. A cage that needs recoating is not a saving; walk away and put the money toward a sound one.
Test the structure the way a bird will
Push and pull each door, including the small feeder doors, and work every catch repeatedly. Doors on used cages often sit slightly warped, closing with a gap a determined bird can work at, and worn catches can be nudged open from inside. Check welds where bars meet the frame for cracks, run a fingertip along cut ends and tray edges for burrs, and rock the assembled cage to feel for frame wobble. Look underneath for bent tray runners and cracked plastic bases, and confirm the grate fits flat so a bird cannot slip beneath it and become trapped.
Ask what lived there and clean accordingly
Ask the seller what species lived in the cage, how long it stood empty, and why it is being sold; a cage from a bird that died of an unexplained illness deserves extra caution. Whatever the answers, treat the cage as contaminated until you have taken it apart, scrubbed every surface and fitting, rinsed thoroughly, and let it dry completely in fresh air before your bird goes near it. Replace wooden perches, rope toys, and any porous accessories from the previous bird rather than trying to salvage them, and inspect metal food bowls for deep scratches and pitting before reusing them.
Know your walk-away list
Decide your rejection points before you travel to see the cage, because a cheap price argues hard in the moment. Walk away for: any flaking or rusted coating, bar spacing your bird's head could enter, a door that will not latch positively, cracked welds, or missing parts you would have to improvise around. A sound used cage from a known home can be excellent value, but only the cage that passes every check earns that verdict. When a measurement or finish leaves you uncertain at the seller's door, owners can put follow-up questions to KinsLe's Kimmy, and she draws her replies from the KinsLe Answers library, so you can decide before you commit.