Hi and welcome to the community!
In order to perform a transaction consistent backup of an SQL server an Application Aware processing
should be used. When you use application-aware processing VSS writers are leveraged so normally if an SQL server is being backed up you should be able to trace SQL Writer Service
activity. Another approach to perform a transaction-consistent backup is to trigger SQL native tools via pre-freeze scripts
. In this case you can track backup-related activity in SQL server logs.