How to See Bokeh

In Existing Images

Look for points of light in the background. Perfect subjects for this are distant points of light at night or sometimes light shining through leaves or specular reflections in daylight.

If they all blend together nicely, that's nice bokeh. If they are perfect little circles, then that's neutral bokeh. If they are all swimmy and look little little rolled up condoms or donuts, then that's bad bokeh. 

If they all are regular polygons that tells you the shape of the lens diaphragm. Yes, you can actually tell how many blades the lens's diaphragm had!

If they are perfectly round in the middle of the image and oval or lentil shaped at the sides that tells you the image was probably shot at full aperture.

If they are all flattened ellipsoids (vertical ovals about twice as tall as they are wide) then that tells you that the image was shot with an anamorphic lens. You'll see this in cinemascope motion pictures, not in still photographs.

In Your Lens

Find a point of light in the distance. You can do this easily at night by finding a distant point streetlight, or you can do it indoors by taking the reflector off of a Mag-Light flashlight and just setting it up on the other side of the room.

Now look at the ground glass as you focus. If you see perfect round disks your lens has neutral bokeh, if you see soft-edged shapes you have good bokeh, and if you see doughnuts you have bad bokeh.

If you see something other than neutral bokeh you'll see the quality of the bokeh change as you focus both in front of and in back of the point of light. Of course you can't usually focus beyond a distant point of light, unless you have a view camera or a lens that allows focusing beyond infinity.