Frame first. A hardwood frame — kiln-dried to roughly 6 to 8 percent moisture content — holds its joints for years; a softwood or particleboard frame is lighter to ship and cheaper to build, but it flexes and can crack sooner, especially at the arms. Ask what the frame is made of directly; a store that can answer in one sentence has actually looked.
Suspension is the part almost nobody asks about, and it matters more than fabric. Sinuous wire springs are the mid-market standard: S-shaped wires running front to back, clipped to the rails. Eight-way hand-tied coils — individual springs tied to each other with twine in eight directions — distribute weight more evenly and hold their give longer, which is why they show up mostly on traditional, higher-end frames. Webbing alone, with no springs, is common on lower, more casual sofas and tends to feel firmer sooner.
Cushion fill runs on a spectrum: all-foam is firm and holds its shape with the least maintenance; foam wrapped in fiber or down softens the feel while keeping some support; all-down is the softest and needs regular fluffing to avoid going flat. Foam itself is rated by density in pounds per cubic foot — 1.8 to 2.5 lb high-resilience foam holds up for 7 to 10 years or more, where cheaper 1.2 to 1.5 lb foam can start softening within two or three.
For fabric durability, ask for the Wyzenbeek rub count: 15,000 double rubs is a reasonable floor for daily residential use, 30,000 and up is commercial-grade. A store that can quote a rub count on request, rather than just "it's durable," is one that has actually read its own spec sheets.