To get the value of the selectionMode attribute of the datatable (primefaces) in MB:
DataTable dt = (DataTable) FacesContext.getCurrentInstance().getViewRoot().findComponent("IdForm:IdDataTable");
String sm = dt.getSelectionMode();
And how do I get the value of the lazy attribute?
To get any attribute of a JSF component you just need to have an instance of type UIComponent of it, which with the method getAttributes() returns any desired attribute by passing only a key to the Map<String, Object> returned.
For your problem it can be used like this:
UIComponent dt = FacesContext.getCurrentInstance().getViewRoot().findComponent("IdForm:IdDataTable");
boolean lazy = (boolean)dt.getAttributes().get("lazy"));