Hi there jesnw,
If you are conducting your visual assessment in Google Earth, there will be a small icon near the top that allows you to scroll through different imagery dates. You can find it next to the icon that toggles whether sunlight effects are turned on or off. Alternatively, it is the 8th icon from the icon on the most right (View in Google Maps).
That should allow you to know the exact date of the base imagery you are viewing in Google Earth, down to the MM/DD/YYYY (e.g. Imagery Date: 12/31/2016). Additionally, you can see the image provider (e.g. Image Landsat/Copernicus).
Hope this helps.