I don't know of any way to check the tension other than with the Porsche tool or the "Kempf tool". From somebody who has crashed all 32 valves on one of those engines you're correct when you say you want to get it right - if for no other reason than to be able to drive without worrying about it. I know Dave Roberts sells the Kempf tool at www.928gt.com
and I think 928 International sells it at www.928intl.com
for $50-60. Check out
under "Engine" for testimonials from people who have used the tool.