how can we identify convex and concave lens without touching them?

if by moving the lens forward and backward the image is erect throughout then it is convex lens and first if it is erect and then diminished then it is concave lens

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Place it on a paper (obviously written) if the image of paper gets magnified >1 (i.e larger than original) then its convex lens else concave(magnification<1)...

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To differentiate between convex lens and concave lens simply use few LASER lights and arrange them in parallel taking some distance and place the lens one by one firstly if the LASER lights converge at a point in the direction of the light then that lens is convex and if the light diverge after passing through the lens then the lens used is concave lens.

Large convex lens.jpg                       Concave lens.jpg

                                 (I)                                                                                                           (II)

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place your finger infront of it......if it gets magnified then it is convex and if it gets diminished it is concaved....HOLA
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