Extend the slide fully and check it moves smoothly and evenly across its whole length without catching or feeling gritty at any point, since slide condition is the single most important factor on a trombone. Check the inner slide tubes closely for dents, even small ones, since these directly cause the sticking and uneven movement that makes a trombone frustrating and eventually unplayable at speed. Check the water key (spit valve) for a tight seal and the tuning slide for free movement. Ask whether the slide has been recently resurfaced, since that's a routine maintenance item on a well-cared-for used trombone that significantly affects how it feels to play.