Major Moodle offers to use module pages with sideblocks and without. Advanced themes need to change the CSS depending on existing sideblocks / not existing sideblocks. Technically Moodle places the "layout-table" when sideblocks are possible.
Best would be to add the class "haslayouttable" to body. Second best would be to add a function "has_layouttable()
{ ... }" which returns true when a layouttabel is placed in the page. Like the function "is_in_popup() { ... }". We are getting near OO notifications here 
.