When looking at a dashboard in Desktop, you can click on a worksheet which will select the worksheet and you'll see four icons pop up (usually in top right corner, sometimes top left corner). The last icon is a downward triangle. Click it and you'll see a long menu with options such as Go To Sheet, Duplicate, and Fit. One option is labeled Filters. Click Filters and it opens a submenu of all filterable fields on the worksheet. If a field is being used as a filter in the dashboard, it will show a checkmark next to the field name.
Note that this is an easy way to add and remove filters from a dashboard. But in your case, you are using it to figure out which worksheet is doing the filtering.
Be aware that dashboards can have filter actions so sometimes worksheets will cascade the filter values to other worksheets on the dashboard.