If they did the master reset with deleting all data, it won't be easy, but there may be perhaps a chance. There may be some traces left anyway. If you did not use the watch, and did not connect it to your account, then it would be easier, because you could go through the individual folders on the watch, and look at diverse logs and data files that might be left even after the master reset.
If you already connected it to your account, or used it, it will be more difficult to distinguish what comes from you, and what was left before, but with some detective work, you may still be able to find out.
First of all I'd look at the file //GARMIN/Totals/Totals.fit with the help of the FitCsvTool. There might be perhaps some device totals left from the previous owner(s). Also folders like //GARMIN/Debug or //GARMIN/Eventslog, //GARMIN/Apps might reveal some traces left.
And if the previous owner did not do the full reset, then most of the folders would contain his/her data - check the folders Activities, Courses, Workouts, Schedule, etc. etc
