How do you determine which geographic coordinate system is used by a GPS device? For example, my Garmin GLO is outputting NMEA data with lat/lon. What system is represented by those lat/lon values? I think I heard the answer is WGS84 but how do I know / verify this? Is it just assumed all GPS NMEA data, unless otherwise projected, is WGS84?
It's safe to assume that it's WGS84, that's the system that the GPS satellite system uses internally, it's the default for the NMEA standard, and in my experience it's always the default system that a consumer GPS is set to use. Some GPS units will let you change the coordinate system that's displayed on-screen but that shouldn't affect the NMEA output. I believe the datum used in the NMEA output can be changed as well but if you've never changed it, it should be set to WGS84. I don't know exactly how you'd check that on your specific unit though.
I haven't seen the GLO interface, but in older Garmin units (Vista HCx), you would access the Setup menu from the main menu and then go to Units menu to see/change your Map Datum.