How do I know if a rollup has run?

You can create an audit report to determine whether all records that should have been updated have been updated correctly.  The columns used in an audit report would include the target (destination) field of the rollup as well as a report formula summarizing the results from the child records. 

For example, assume there is a rollup on Account summing the total Amount of all related Opportunities in a field called 'Total Amount'.  If I wanted to ensure that my rollup was updating all records correctly, I would create a Summary report as below, where the left column is the rollup field and the right column is a report formula using the summed Amount field from Opportunity :



You can also verify that there are no Rollup Helper jobs still processing in the Apex Jobs list by looking at your apex jobs and ensuring that no Apex jobs or Future Apex jobs are processing.

To view your Apex Jobs:

1. Go to setup page

2. Under the monitoring section select Jobs > Apex Jobs

3. Ensure view is set to All

4. Batch Apex and Future Apex are the Jobs that process Rollups, if there are not any Batch Apex or Future Apex Jobs processing then your Rollup has finished running.

Notes:

Do not be alarmed by the large number of aborted Scheduled Apex jobs, these are necessary for Rollup Scheduling. Scheduled Apex jobs abort if there are no Rollups scheduled at that moment in time

There is not a way to identify specifically what Rollup a job is currently processing