Hi David--
It depends on whether you are accessing it via dlgitem_* functions or via the XUI document structure directly.
If you are using dlgitem_* functions, you can do it by setting an ID on the <tabbox> element, and using
local tabindex = dlgitem_get_value(dlgwin, "myTabBoxID")
where dlgwin is the window ID of the dialog, and "myTabBoxID" is the ID string you assigned to the <tabbox> element. It will return an index indicating which tab is selected (by count, e.g. "1" for the first tab, "2" for the second, etc.).
If you are using XUI structure, you can just use the usual document interrogation methods:
local oids[];
oid_find_child_attrs(oid_root(dlgdoc), oids, "id", "myTabBoxID");
local tabboxoid = oids[1];
local tabsel = oid_attr(tabboxoid, "selection");
--Clay