There's no easy way of telling if a text has been clipped.
This clipping is done on Labeled objects, and in the implementation of LabeledSkinBase, we can see that all the logic for clipping is delegated to computeClippedText(), which returns a (clipped or not) String:
result = Utils.computeClippedText(font, s, w, truncationStyle, ellipsisString);
[...]
text.setText(result);
This method does not get a reference to the Labeled object and does not fire any events, so the only chance we have of whether computeClippedText() decided to clip is to have a look at the actual Text node (text), which contains the actual text being shown. Since Labeled doesn't expose this node, we have to resort to doing a lookup:
String originalString = myLabeled.getText();
Text textNode = (Text) myLabeled.lookup(".text"); // "text" is the style class of Text
String actualString = textNode.getText();
contentHasBeenClipped = originalString.notEquals(actualString);
And there we have it!