Just had an after-thought, why just have a problem with ships...what about techs and trade goods? If you know what weapon techs they have you have an avenue to build w/o looking at their ships.
Yep, I use that as an early exploit, before I have ships anywhere near another faction's part of the map. But the trade window won't show a tech they have, that you also have. So it's not 100% reliable for knowing what the other faction is doing. As a practical matter, I usually have trade routes set up with all the races by the time I start building military ships, along with a few sensor scouts, and I can use them to scout out what weapon/defense branch the other side is building, if not the specific tech level.
I'm not sure what a good fix for that would be... although it might make sense that you'd have to have a certain degree of good relations to get access to that info... maybe something above neutral. I certainly can't picture a hostile or wary faction sharing ship intel with an enemy.
That would seem logical, but it wouldn't stop sneaky, unscrupulous behavior.

I've been known to be best buddies with a race, right up to the point when I stab them in the back with an invasion.