Ever have to recreate a document from an Acrobat PDF? You can find out most everything about the text by using the Object Inspector, except the leading. Well, here's a cheesy way to figure it out. Open the PDF in Illustrator (you just need one page). Release any and all clipping masks. Draw a guide at the baseline of the first line of text, and one on the line below. Now, Option-drag the first line to make a copy, and position it exactly next to the original first line at baseline. Then put a return anywhere in the copied line. Now adjust leading of the copied lines, so that the second line of copy rests on the baseline of the second line of the original. Now you know your leading.
Or you could buy expensive software to find the leading. Your choice.
From Greg Ledger