How can we distinguish between a concave and convex lenses without touching the lenses?

Concave and convex lenses can be distinguished just by looking at them. A concave lens bends inwards and is thinner in the middle. A convex lens bends outwards and is thicker in the middle.  

Concave and convex lenses can be distinguished further by looking at the images that are produced when an object is looked at through the lenses.

Rays of light that pass through a convex lens converge. This means that the rays of light are brought closer together. The rays of light that pass through a convex lens all come together at one location. This location is called the principal focus. Images produced by convex lenses are magnified.

Rays of light that pass through a concave lens diverge This means that the rays of light spread outwards and apart from one another. As a result, images produced by concave lenses are diminished. This means that the images produced by concave lenses appear smaller than they really are.