Get many GPS traces/points maybe even from different devices and at several positions (to avoid single [gps reflections or shadowing](https://wiki.openstreetmap.org/wiki/DE:Genauigkeit_von_GPS-Daten)) of the target area to be sure. Note that the offset could change, especially in mountainous areas. You could ask the original creators what the source of the OSM objects is (if they have no source tag and no changeset comment) and if they remember aligning the aerial imagery before tracing. And then I would move (that is what I do currently) single, small groups of streets/buidlings if I am editing there anyway and notice a strong misplacement. No moves of a whole suburb. Ensures that you do not make a too big mistake if the move was not correct for some reason. In quite some places in my mapping area I have concluded that the OSM objects are traced from misaligned aerial imagery (which e.g. the old bing (pre summer 2012) was in my area).
Also see [bing-imagery-problem](//help.openstreetmap.org/questions/11380/bing-imagery-problem) or [alignment-of-track-vs-background-imagery](//help.openstreetmap.org/questions/6385/alignment-of-track-vs-background-imagery) (found [via google](//www.google.com/search?hl=de&q=misaligned+site:help.openstreetmap.org)).
