Some imagery tiles provide metadata, including date of acquisition, for each tile. This information is available in both JOSM and iD (Shift-Ctrl-B) for at least ESRI layers. In some countries there are also other historical layer layers which may help. For instance os.openstreetmap.org [os.openstreetmap.org][1] shows various issues of Ordnance Survey StreetView Open Data from 2010-2016. This can assist in more accurate ageing of imagery layers. When one of Bing Streetside, Mapillary or OpenStreetCam imagery is available these can also help. Most of the time relative age needs to be inferred from imagery itself. In many places this is self-evident, for instance, buildings on what was farmland, but in town centres and long-standing residential suburbs the signs are more subtle. One clue I'm aware of is presence of rooftop solar installations can be a good guide at present because many of these have been put in since 2010 and in many areas there are enough to discriminate between other very similar imagery.imagery. [1]: http://os.openstreetmap.org

Some imagery tiles provide metadata, including date of acquisition, for each tile. This information is available in both JOSM and iD (Shift-Ctrl-B) for at least ESRI layers. In some countries there are also other historical layer which may help. For instance os.openstreetmap.org shows various issues of Ordnance Survey StreetView Open Data from 2010-2016. This can assist in more accurate ageing of imagery layers. When one of Bing Streetside, Mapillary or OpenStreetCam imagery is available these can also help. Most of the time relative age needs to be inferred from imagery itself. In many places this is self-evident, for instance, buildings on what was farmland, but in town centres and long-standing residential suburbs the signs are more subtle. One clue I'm aware of is presence of rooftop solar installations can be a good guide at present because many of these have been put in since 2010 and in many areas there are enough to discriminate between other very similar imagery.