In the upper right-hand corner of the open schedule or schedule skeleton, click on the drop-down menu next to "Mode:".  Select "Show shifts given to."

When you select a volunteer's name from the list on the right, the days they are scheduled will appear in green in the calendar on the left. When you click on one of those days, the shift and their name will also be highlighted in green so they can be easily located.

You can also access this view by going to the View menu at the top of the window, and choosing the option "Show shifts given to..." or by clicking on Ctrl+F.