To check the Viva Engage(Yammer) Network mode :

Login to the Viva Engage(Yammer) Portal as Global Admin. Go to Settings -> Edit network admin settings -> Security Settings.

Check the Office 365 Identity Enforcement as shown in the below picture.

Non-native mode : Office 365 identity and Office365 Connected Viva Engage(Yammer) Groups settings will be on disabled state.

Native mode : Both Office 365 identity and Office365 Connected Viva Engage(Yammer) Groups will be enabled, and M365 Native mode property should be True.

Hybrid mode : Both Office 365 identity and Office365 Connected Viva Engage(Yammer) Groups will be enabled, and M365 Native mode property should be False.