A good pair of flared trousers has weight to them before you've even stepped in, that's the first thing I check. Pick them up and the fabric should fall rather than flop, with enough body that you can feel the flare wants to hold its shape, not just hang there like a bit of curtain. Cheap polyester mixes feel thin and static in the hand, and they'll feel exactly the same way on your legs by lunchtime.
Wear is the real test, and this is where I get a bit obsessive about fibre content. A heavier cotton twill or a wool blend softens at the knee over months and starts to move with you rather than against you, which sounds poetic but is just physics, really, fibres relaxing under repeated stress. Cheap viscose does the opposite, it goes thin and shiny at the seat within a few wears and never recovers. Wash them inside out, on a cool cycle, and hang them by the hem rather than the waistband, because a peg mark across the front of a flare is obvious from across a room. Iron the crease if there is one while they're still slightly damp, it holds for days that way instead of hours.
They're brilliant for exactly the situations where you want to look like you've made an effort without anyone being able to say what the effort was, an early dinner, a work thing with wine involved, a Sunday where jeans felt wrong but you still wanted to walk properly. Autumn and winter is when mine earn their place, under a long coat with the hem just skimming a boot, because that's when the shape actually reads as an outfit rather than a costume. In high summer they can feel like hard work, heavy fabric against bare legs in thirty degree heat is nobody's idea of fun, and I've stood sweating through a heatwave in a pair that looked wonderful and felt like wearing a tent.
They are also, honestly, the wrong choice if you're spending the day properly moving, cycling, chasing children, anything involving stairs at speed, because the hem catches on everything that isn't flat ground, and I have ripped one pair clean off a bike pedal and never quite trusted that style again. But on the right afternoon, with the right height of shoe underneath, there's a particular moment when you catch your reflection walking past a shop window and the whole leg moves as one clean line, and that's worth the ironing.