Domestic travel does no have free checked bags for economy. Elites and premium cabin travelers have bag allowance waiver.

If the appropriate card was used to purchase the itin and the card is still valid at travel, you will get 1 or 2 bags for free (depending on the card).
If after signing up for the card, trips taken in the 90 days after the sign up, even if the card not used, will also get the same bag fee waiver.

Unfortunately the online itin does not always reflect the waiver but at check-in things seem to be more complete.
Mileage Plus Branded Credit Card and Checked Baggage Fees