I don't know about getting foresight from the dead.
Sauron, as a
Maia, should have more foreknowledge than the spirit of a dead man or
elf. But I will get to this below. The gift of foresight could come direct from
Iluvatar, or second-hand from someone else who had seen the future...
Melian or
Ulmo for example.
If the
spirits of
men or
elves have any true knowledge of the future, and a necromancer gets this from them, I would doubt that it really foresight. I think it would be more like going to a fortune-teller that seems to speak the truth... you hear of the future from someone, but do not see it with true foresight.
I also had the impression that necromancy was just some art of dealing with the dead... and not necessarily doing this to learn of the future.
But if someone thinks Sauron was able to get knowledge of the future from the dead, then I'd be interested in hearing how it is that the spirits of dead men get this information... are they free to
travel in time within
Arda or do they get some amazing education through contact with other spirits such as
Ainur? I doubt the spirits of men and elves that stick around Arda would have too much contact with Ainur. Perhaps only
Mandos for a time? But any spirit that sticks around wandering Arda has refused something... and is not on the right path... so their communication with Ainur spirits seems very unlikely to me. But, if the spirits of the Children can actually travel in time within Arda and see the future, then perhaps they could transfere that vision direct to a living spirit, but as Inder has shown, these visions could not be trusted... and if they are decietful they are not foresight.