How can I tell that?

I think I saved the game at the invasion point, but what info or place on the screen do I look at for that datum?

All I could see - or so I thought - was that all the squares had been converted, and that all the squares had the Industrial Sectors finished.  I deduced that they must therefore be building a ship - as there were no squares left to convert or buildings left to build.

That's clever (!), but I don't know the cost of the DreadLords' ship.

That is, i could get a sense of how much production had been done, but will that help me tell how long time-wise before completion?

I had the Dreadlords pop up in my current game, unfortunately well out of range in unexplored territory.  My first encounter with them was seeing a Dreadlord Dreadnaught with 498 attack appear at the edge of my territory -- my best ship at that point was a medium hull with 4 attack and 4 defense, and tech speed was Very Slow.

That made for a pretty intense game

The game up to that point was focused on control of resource bases -- I grabbed quite a few in the early game and rushed for Starbase Fortification so the bases could soak up the brunt of enemy attacks.  I was playing a custom race with the Arcaean tech tree so I had the Lite constructor module.  I was at war with the Terrans who were closest to the Dreadlord planet, at the far end of a Gigantic galaxy.

When the Dreadlords appeared they hit the Terrans first.  They only had a few ships, that Dreadnought and a handful of frigates.  Even the frigates had over 200 attack but only 20-30 hp.  But they mowed down my dozen plus starbases in short order.  Losing 4 maxed-out Econ resources really does a number on your economy

With my Navigation Centers and stolen Impulse Drive I could pump out multitudes of fast, cheap tiny hulled Lite Constructors and go around rebuilding the resource bases as fast as they were destroyed -- the one good thing was that the DL ships were slow.  I didn't bother arming them when I rebuilt them, just got enough bonuses flowing to keep afloat.  I put everything into research and got Star Federation and Stock Markets so my economy could survive the intermittent loss of the econ resources.  After that it was time to research some better ships.  I got a couple very lucky Creativity hits and soon had Discovery Spheres.

After that it was just a matter of researching Huge hulls and enough weapons, defense, and logistics so that a fleet of 3 huge ships with one tiny hull fleet defense module ship could take out a DL Frigate.  It helped that the DL were at war with everyone, their planet was constantly besieged.  After I built up my forces I brought up some fast constructors, built a starbase right beside their planet so I could take advantage of the Advanced Fire Control module that only works within your own influence, and attacked after a particularly heavy round of combat between them and the Drath when the DL defenders were at their most damaged.  I got a class 39 planet with Industrial Sectors on every square (that just about killed my economy right there) and stole Psionic Beams when I invaded.  A few of those on my huge hulls and I was the new Dreadlords as far as the rest of the galaxy was concerned  

And I never did have to deal with that dreadnought.  To my vast relief it just disappeared when I took the DL planet, instead of going pirate or joining another race as I'd feared.