Under Flight Availability, you can search for inventory on your flight. J3 means three seats for sale in full-fare business (discount business could be the same number or less), etc. W0 means zero seats for sale in premium economy (because W is full-fare there, discount premium economy would also be zero).
If your tickets are valid but they can't assign seats, they've come close to admitting it unless they offer another explanation. Sometimes seats are blocked rather than occupied, to be allocated as needed at the airport (for pax with disabilities or traveling with infants); in this case, it might not be oversold, but ExpertFlyer should show blocked seats differently from occupied ones.
Note that the seat map is not definitive for this. If the inventory says J0, then the cabin is likely full even if some haven't chosen seats.