[OSM-newbies] aerial imagery vs OSM data - which is correct?

[Alexandros Papadopoulos]

Hello

Sometimes aerial imagery is not aligned with existing OSM data. Is it
possible to *know* which of the two is right?

I was going by the assumption that OSM data always trump aerial
imagery, but after finding an instance where this was not the case
(because the OSM data had been previously traced on a misaligned
aerial image) I'm not sure what to do.

If I was lucky enough to have been through that area AND had the GPS
on, then it's fine - I have a trace I can trust and I'll align
accordingly. But when one hasn't got a personal trace, what can be
done?

Thanks

Alex