I believe you'll need to study and then book an exam.
Book your exam here:
https://www.asltasman.com/TOL/Genera...sponsorid=8709
ASL use computer based exams now, I sat one last week.
You go into a room full of computers, you start your exam and answer multi choice questions there.
You can go back and forth between questions and there's a finish button at the end.
You get your results once the time runs out on your exam from the ASLTasman website.
If you're looking for some material to study then Mark Woodhouse has a good ATPL Air Law study guide from :
Aviation Waypoints - courses
His material is very much related towards passing the ASL exam.
Learning ATPL air law in a book is very much different from experiencing it in a jet and I'd imagine it wouldn't be too hard for you since you already have your JAA ATPL.
All the best!