Sometimes they are marked in very small text; other than that the only other way to tell is to have it measured.How do you tell which diopter is installed on your camera?
I have a -1.5 for the RB Waist Level finder that is clearly marked on the plastic surround in white text as -1.5; but I have a second one that is supposed to be -1 that does not have any markings. If you are talking about a diopter for the prism type finders that do not have a plastic surround but just a piece of glass or plastic they are typically marked with dots on the side of the glass to indicate their strength; I have one with 2 red marks on the side and is supposed to be -2 and I guess they may use some other color to indicate + diopter?
Basically a diopter measurement is a measurement of the power of a lens and is the reciprocal of the focal length in meters.
It basically moves the focus forward or backwards in oder to correct for your eyesight.
Negative diopters are for people who are nearsighted and positive ones for people who are farsighted.
Reading glasses are typically positive diopters.
I don't think I have have ever heard of a camera diopter lens of 11.5; I think that measurement may be in arc minutes.
Maybe there are some optometrist folks on apug that can go in to more depth or correct me if I got something wrong.