On the actual map, next to the fleet symbol, I think it would be useful to have next to the fleet marker a 2 digit number showing how much delta v is left in the fleet, and besides a number showing how much delta v was spent last turn.
So, for example, you would have something like below:
So, next to the fleet symbol, the 5.6 would be how much delta v (in km/s) is remaining, and the (1.0) says that it burned 1.0 km/s last turn. I think this should be visable on both friendly and enemy fleets: it should be relatively easy enough with basic survailance to determine how much delta v an enemy fleet has left, and how much of a change in velocity a fleet has burned. This would be especially useful in missile and other intercept dodging to see how much delta v is being expended on dodging.
Hm, I think basic intelligence would give one a fairly good idea of how much delta v a fleet has. Besides having actual plans for enemy ships, simple volumetric measure of a ship would give a good rough approximation of delta v left. If you had a more complicated "intelligence" functionality, one could simulate some of the fog of war of this: have hidden stats for a never before seen ship, get some rough range revealed as time is given to observe the craft. Maybe capturing a craft mostly intact gives one better info (the selective damage missions) or pefect knowledge in those situations where there's a mole.
But, given the set up of this game, I think assuming full knowedge of the other fleet's delta v reserves is close enough to accurate and vastly simplifies things.