Maya :: How To Detect Whether Program Is Running In Batch Mode

Situation: Rendering a sequence of frames with different image dimensions (photos of 3D scene for printing, different image dimensions needed for different purposes). Changing image width/height in pre-render frame MEL script. Therefore, need to know what frame we are on.

Unfortunately, mental ray batch render returns frame number (currentTime) of PREVIOUS frame when called during pre-render frame script. I have to add 1 to get the frame that is about to render, to determine needed image dimensions.

This is different than rendering current frame within Maya to preview what will happen -- currentTime is already on the correct frame.
 
So, in my frame mel script, I need to know whether in batch mode or not, to know whether to add 1 to time.
 
How do so?
 
I searched docs and googled.  All I found is a mention "Implementors of render traversals may wish to check whether they are running in batch mode", that did not say how to do so. And a mention in 2006 of "about -batch" that supposedly gives documentation that mentions how to do so. But the about command must be obsolete.