The moment I notice a modern sofa doing its job is on an ordinary Tuesday, when you drop onto it sideways with a plate on your knee and nothing hits a hard edge or wobbles under you. No fuss, no skirts or fringe catching the vacuum, just a clean shape that holds its line against a wall and still feels like something you can sink into after a long day. That's the whole appeal to me, it looks composed when nobody's touched it and still works like a regular couch once someone has.
If you're checking one in a shop or the moment it arrives, get your hands on it before you worry about the color. Push down on the seat cushions and feel for the frame underneath, you want a firm, even resistance, not a hollow give that tells you there's not much structure holding things up. Lift a corner slightly if you can, a well built frame won't creak or flex. Run a hand along any exposed wood or metal legs and check they're attached solidly, not just screwed into a thin baseboard that will work loose in a year of being bumped by vacuum cleaners and shins. Check the seams are straight and the fabric is pulled tight with no puckering, especially on the arms where modern designs often taper into a slim, low profile that shows every flaw in the upholstery.
The difference between a sofa that holds up and one that disappoints usually comes down to what's inside it, even though you'll never see it again after delivery. A hardwood frame, kiln dried so it won't warp, with corners that are glued, dowelled or screwed rather than just stapled, will outlast a particleboard frame by decades. Cushions filled with a foam core wrapped in fiber keep their shape far longer than foam alone, which goes flat and starts looking tired within a couple of years. Ask what's inside if it isn't listed, a decent shop will know the answer without hesitating.
Over time, rotate and flip loose cushions regularly so the wear spreads evenly, and keep it out of direct sun if the fabric isn't treated for fading, since modern designs in pale neutrals show sun damage fast. Vacuum the seams every so often and treat any fabric protector reapplication as routine maintenance, not an afterthought.
If you're starting from nothing, go for a mid sized three seater in a neutral, performance weave fabric. It flexes with whatever room it ends up in next.
A good modern sofa should look almost too simple to be comfortable, and then prove you wrong every single evening.