I was never aware of this before, but I just tested the same thing and the screen-value does strip trailing spaces (but not leading spaces). I just looked in the online help and found the following:
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PROGRESS truncates trailing spaces in character fields. If a character field contains only one space, PROGRESS truncates the space to a null value. You can use the TRIM function to truncate leading and trailing spaces.
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Why do you need to know if there are trailing spaces or how many? If I knew this, maybe I could suggest an alternate method for solving your problem.
-Chris