According to the COMREF at least, there are no commands to dectect screen resolution.
There are ways to get it though. You could, at the start of your mission, have a dialog that reqests the player select his monitor ratio/resolution from a drop down list. Then, depending on which resolution they selected and which options you offered, you could have "_x" number of dialogs, one for each of the resolutions you support. Note that some people, myself not included, don't know generally what "ratio" they have, or what resolution, so you will probably want to walk them through it, and explain that a CRT is commonly 5:4 and that widescreen displays are 16:10 etc.
That is probably the most stable way of doing it. Unfortuately, it will require more work on your part.