When spectating a game it is currently not possible to determine what pokemon were placed on the bench or active spot in the game log.
Would it be possible to add what was started with for completeness.
Done, coming with the next game update: after the setup, once both players’ Pokémon are turned face up, the game log now says what each player started with, for example “Ash starts with Pikachu (BS 58) as the Active Pokémon and Bulbasaur (BS 44), Charmander (BS 46) on the Bench” (card names can be hovered). Spectators who join later see it too. Thanks for the suggestion!