How can I identify markets that accept passive orders via the Betfair API?

In some in-play markets, orders that won't be immediately matched are placed into the market without being subject to the usual in-play bet delay.

You can identify markets that have passive orders using the betDelayModels field returned via listMarketCatalogue, listMarketBook, listRunnerBook.
 
See below details of recent API updates relating to this from the Betfair Developer Forum

• New-field-betdelaymodels
• Dynamic-bet-delay-model-added
• Api-release-betdelaymodels-to-listmarketcatalogue-response
• Betdelaymodels-added-to-marketfilter

Darts, Basketball, Tennis and selected football markets are now available with this feature. 

You can read more in recent and past updates relating to this feature in the Betfair Exchange Newsletters - see https://betting.betfair.com/betfair-announcements/exchange-news/