I'm looking at buying a ticket on Delta for an upcoming trip (NW doesn't go there, no award seats available).

My understanding is that I can fly on Delta and have the miles credited to my WP account. I'm told that it's NW who determines which flights qualify for mileage credit. I'm also told that the Delta agents are clueless about such things and I better figure out the details myself rather than relying on them.

So, can one of the cognoscenti here point me to someplace where I can find what fare class designations will (and won't) qualify for mileage credit on NW?

Thanks!