SQL Server 2008 commit overhead warnings
Using performance monitor in windows you can for example watch the redo queue.
Also any other db monitoring tool would help.

The next sentence explains the redo queue, since I can't do it better than msdn here the quote:
"The mirror server immediately writes the incoming log to disk, where it is held until it is applied to the mirror database. The log waiting on the mirror's disk is known as the redo queue."

( https://technet.microsoft.com/en-us/library/ms189901%28v=sql.105%29.aspx )