The service trace log file contains very detailed information if you are debugging an error/issue. The trace log files are located in below folder.
In MSRS12.MSSQLSERVER , MSRS12 is version of Reporting service such as SQL Server 2012. MSSQLSERVER is instance name. In your environment, check as per your Reporting version and Instance name.
C:\Program Files\Microsoft SQL Server\MSRS12.MSSQLSERVER\Reporting Services\LogFiles
As you can see that the new file is created daily for log information with timestamp.