Try to discern whether the internal construction of the furniture is good. This can be difficult without removing the leather, but there are a few general indicators. Feel the frame wherever you can through the leather; a thicker frame tends to impart more strength to the overall piece. Lifting it to gauge the weight can also help, though it is by no means an absolute rule; in general, better-built furniture tends to be heavier. Gently flex the piece to see if it holds up well against pressure. Check the external construction as well; visible joints and seams can give an indication of how the piece was constructed, and how well it is likely to last.
If the furniture piece is used, inspect it for overall wear and tear. On leather furniture, the biggest issues are holes and rips in the leather, as well as stains. All of these can be difficult, if not impossible, to remedy without simply replacing the leather. Also check by feel for any spots where the leather has become significantly softer than the rest of the material (these will be on the most heavily-used part of the furniture). These soft spots will have to be treated gently, or they can quickly turn into rips or holes.
Look at the overall condition of the leather. Is it shiny and smooth and well-cared-for, or dull and rougher? In used pieces, ask how long it has been since the leather was last conditioned; most pieces should be conditioned once or twice a year. If you can, determine how thick the leather is; thicker leather will be less prone to tear or crease. If the piece is new, ask what the recommended care procedures are for each model, as they may be different.
Because of the many imitation leather products available, it is sometimes possible that a piece of "leather" furniture is not made of leather at all. Any reputable furniture dealer will label its stock accurately, but some unscrupulous or ignorant private sellers may claim a piece is leather when it is an imitation. The easiest way to tell is usually by looking at the back of the material, which may require looking at the underside of the furniture. Most imitation leather products have a cloth mesh backing that real leather does not have, so if the back of the material does not also look and feel like leather, it probably isn't.