To briefly supplement my GPS Device Analysis research, the GPX file itself should have interesting metadata qualities that can help you decide its relevance to your case. For example, I was able to pull the following data out of my GPX file using ExifTool by Phil Harvey:
Gpx Metadata Link Href : http://www.garmin.com
Gpx Metadata Link Text : Garmin International
Gpx Metadata Time : 2013:03:03 19:38:00Z
Gpx Trk Name : ACTIVE LOG: 03 MAR 2013 14:11
Gpx Trk Trkseg Trkpt Lat : 11.111111
Gpx Trk Trkseg Trkpt Lon : -11.111111
Gpx Trk Trkseg Trkpt Ele : 143.85
Gpx Trk Trkseg Trkpt Time : 2013:03:03 19:35:50Z
Gpx Trk Trkseg Trkpt Extensions Track Point Extension Speed: 8.24
Gpx Trk Trkseg Trkpt Extensions Track Point Extension Course: 292.24
This provides yet another quick way of determining whether the GPX’s last Active Log corresponds with your working timeline.