Are you trying to confirm that a C2C ID sync file successfully onboarded your IDs? This article may help.
ID Sync files create automatically generated Synced Audience traits which reflect the number of synced IDs under the Cross-device ID option. Unlike trait onboarding files, there will be no record in the onboarding status report. Follow these steps to locate your synced audience trait:
Identify the Data Source name for the onboarded IDs
Navigate to Audience Data Traits
Search “Synced Audience” or your Data Source name, and locate the synced audience trait for your particular data source
Select “Cross Device ID” in the drop-down in the top right. Any successfully synced IDs will reflect within this trait.