How did Arwen see it......

[Daniel Telcontar]

The Palantiri could only transmit images and the thoughts of the other viewer (and then only with his approval).

All Elven mothers had the ability to, to a certain degree, to see the future of their children; therefore the name that a mother chose for her child was considered important, because it had significance to what the child would become.
This does not go for unborn or would-be children though, so there is no chance Arwen can see her future son in my opinion.

The Valar definately used telepathy (they did not have a language of their own and used Quenya when speaking with Elves) but I am unsure if this was a possiblity possessed by Elves.
Estelyn, which Elves would you say had the power of telepathy? I assume Galadriel had to a certain extent, but I believe that was rather Nenya than any innate power that she possessed.