How to know if a Label has been tail truncated?

I set MaxLines on a label to 5 and LineBreakMode to TailTruncation. How can I know if the text was indeed truncated so that I can show a "View More" button? Note that I'm setting FormattedText since I have Spans with different colors in the text. When I look at FormattedText in the debugger, I still see the original text.