The easiest way to check if the external timebase is being used would be the following. Go to Measurement and Automation Explorer and open a test panel for your PCI-MIO-16XE-50 card. Select Edge Counting in Counter I/O and then select the appropriate clock. This will count the edges of the selected clock. If you change the external timebase to a slower clock the test panel should account for this, by counting much slower.
