Each time a process runs it creates a detailed Process Log. This includes a trace of all API interactions with Workbooks and messages logged by Scripts. There be many thousands of Process Log records for a process which is scheduled to run frequently; you can use filters on the Exit Code and Summary columns to find the logs for times where a process failed.

The Summary column is populated with the last entry which was written to the log for each run of a Process.  The Exit Code should normally be 0 (zero) if it was completely successful or 1 if it should be retried later or has warnings.