Theoretically, you should receive an email with the info, but it rarely if ever works that way.
I usually call to confirm that the upgrades went through while on my way to the airport. Since they introduced the system I am 12 for 12 on upgrades, even though one of 'em only came at the airport.
Alternatively, on NW code itineraries, you can pull your itin up on the web at nwa.com or on a WAP enabled phone and verify the upgrades.
"The A340-300 may boast a long range, but the A340 is underpowered" -- Robert Milton, CEO - Air Canada