When the startup method of your widget is called, the entire form is loaded in the DOMTree, and all widgets are instantiated. To get notified when a widget is completely loaded, you can add an onLoad handler:
This will invoke the handler once the widget is loaded, or immediately if already loaded.
A widget declared with mendix.widget.declare has an isLoadedRecursive method, which also can be used to delay a handler until all widgets in a given DOMNode are loaded.
If you have an array of widgets which should be loaded before invoking a handler, you can also use:
If you mean by page the HTML page itself (index.html), you can use mx.addOnLoad(function)
If you mean a form, no, but you can use mendix.lang.runOrDelay(actionfunction, conditionfunction) to postpone some code until a certain condition evaluates to true.
You can check if your widget is in the DOMTree and visible by using dojo.marginBox(this.domNode).h > 0