Smell it.....and look for an edge....perhaps in a hidden seam....if the cushions are zippered.
And remember that many lesser grades of "leather" are leather where you sit and plastic on the sides and back....and after a bit the colors don't match.

If it looks really smooth and shiny, it may be what they call bonded or bi-cast leather. That is a thin leather layer with a top layer of vinyl. That is called "leather" but it it doesn't wear anything like leather -- it wears like the plastic top coat. If the back and sides have a different grain and feel, they can be vinyl or they could be splits -- the underneath layer of the hide that will not have a natural grain to it. It can be smooth or have a stamped grain.

Think about how leather feels and wears in shoes, a purse, wallet, etc. It will tend to have a little give where it has been used, but be soft and durable.

Linda and Lascatx,
thanks for the info. I will go and take a look and see if I can tell.

Usually there is a tag that says the materials used - and it would say "genuine leather". Look on the bottom where the tag is often stitched into the liner fabric.

Open a zipper and look at the back. The back of real leather is suede.

Pinch it, and if it moos, it's real!
;)