It's actually very east to check Static GPS Accuracy VS Map and Stability.
Set the unit Navigation to OFF Road (This will switch off Snap to Road) Switch to Norh Up and Switch off 3D Terrain
Walk to an intersection and Zoom + to Max.
Try this at Various Locations.
Of Course if the Map is wrong, then the test is meaningless but it will give you a good indication.
I've tried on 3490 and it's usually spot on, If I cross the road I can see it on the GPS Screen