In GXT 2.2.5, there is a method on store: store.getSortField() which gives the field on which grid is currently sorted. I don't see this method in GXT 3.0.
I'm hoping I actually don't need to care if I can get this implemented correctly.
Our grid filters are separate from the column headers in that they occupy a toolbar above the grid and a user clicks "Search" or "Reset Search" buttons to filter the data. I have a way to set the value of the filters based on the values entered by the user in those filter fields, but I have yet to discover a way to fire a "Search" event to the grid or its supporting cast to tell it to execute a filtered query.