The easiest way to check is by looking at the lower-right corner of the main menu when first launching the game and seeing if the Steeltown logo is there.
If the logo is there, you’re good to go. See this article for information on how to get to Steeltown in-game.
If the Steeltown logo is not there, check your purchase platform to ensure both the base game and The Battle of Steeltown DLC are owned and activated on the same account. Please note that while the base game of Wasteland 3 is available via Game Pass subscription, The Battle of Steeltown is a paid expansion.