"A double concave lens is the one which has two concave surfaces. Now concave surfaces are the ones which are curved inwards. So, a double concave lens will have two inward curved surfaces. The following diagram shows the double concave lens. (Provide the concave lens image which I have attached)
Similarly, a double convex lens is the one which has two convex surfaces. Now convex surfaces are the ones which are curved outwards. So, a double convex lens will have two outward curved surfaces. The following diagram shows the double convex lens. (Provide convex lens image)
Identifying by touch: When you touch the lens and you feel that the surface that you are touching is bulging or curving outwards then that lens will be a double convex lens. If you are touching the surface and you feel that there is a dent or a curve which is inwards then the lens is a concave lens.