Not sure what for you would be the easiest way ; you can look at the actuals against the assignment in OWB for example, look at the timesheets themselves for the resource, perhaps get a time-scaled view in the web-interface to show you where the actuals were incurred, could query the timeslices (timeentry or actuals ones).
I'd probably quickly query the timeslices myself because I am comfortable with the data-model to see the relevant date period and then go and look at the timesheets. If you are not comfortable with the data-model then I'd probably just look through the timesheets to find it (probably an adjusted timesheet if the assignment actuals are zero)