How do you prove you were under duress when signing your separation agreement?

It depends on the facts, but in these circumstances it will likely be challenging for your Ex to successfully claim that she was under duress at the time your agreement was signed, to the point where it should be considered void.

For a court to find undue duress, sufficient to overturn your separation agreement, it will have to look at several factors such as: 1) the nature of your relationship; 2) whether you exerted undue influence on our Ex; and 3) whether you had independent legal representation.  In this case, the fact that you both had such separate advice, and that your Ex signed a document attesting to the fact that she was not under duress, will bode in your favour when seeking to keep your separation agreement intact.