Sometimes what I will do to save a turn spying is to spy into their relations, then do a search on their defensive allies to get their networth. Once I have their networth, i will go back and review my old spy reports (I save them all) to see if I can find one that is around their networths. If I can find a close match I will take their military strength (including jets as turrets just in case the ally is all turrets), and use that as my estimation of my target's allies defense.

As a Demo techer, I understand you reluctance on spending turns spying. I have a lot of spies to make sure I keep failing to a minimum. I also don't try to spy into a target more than twice if i fail, just move to the next.