So, I am building an optical system and have got a Thorlabs plano-convex lens (part # LA1172-C) with a 400mm focal length. This makes the convex side of the lens so flat that it is difficult for me to discern (by naked eye) which side is convex on the lens. I need to know which side is convex in order to build my optical system. Is there a good technique to figure out the convex side of my plano convex lens?
If you are not concerned that it would damage the lens, you can put the lens on a smooth and flat surface and try pushing from the edges. I suppose you should be able to detect a small movement if the convex side is at the bottom.
As for an optical solution? I am not sure. As planar-convex lenses are non-ideal, you could try setting up some equations and equipment to see what the focal points are and compare them to find which side is planar. But using a simple mechanical setup is easier than optical experiments and equations.
In general in thorlabs lens there is a little > pointing the curved surface or written with a pencil or printed in the side of the lens.
If not, in general people place the lens under a long rectilign neon lamp and look at the reflexion on both surfaces. Is the neon reflexion seem curved on one of the surface? This is the curved surface. With a 400mm focal lenght it is feasible, with a focal lenght higher than 1000mm it is becoming more complicate to see the difference.