Currently with XCTrack there is no way to detect if the glider is going forward or reversing. For example if you are 1000 meter about the ground in big wind, and XCTrack tells you that your speed is 6 km/h, you really don't know if you are actually going forward or backwards.
The only option right now is to try to zoom the map a lot and try to see your track on the ground. But if you are in the above situation your last idea would be to fiddle with your instrument.
I recommend to detect direction somehow (compass sensor, or just some clever logic on GPS data) and make the ground speed widget colored in red (either text or background).