I think you're supposed to know, when you're scolded after-the-fact.

Next best thing: download GPS traces at the same time as loading map data into JOSM; tick the checkbox.  Find some heavily-GPS-traced main arteries, and adjust the offset for that imagery in JOSM until those line up.  Preferably, do this every time, rather than following the lore that "it's always 3.14 2.78 in this town".