Its actually quite easy to find out. I have Google earth PLUS and in the status bar at the bottom it tells you the date of the imagery (for Digital Globe).

- GE_Status_bar.jpg (20.44 KiB) Viewed 8432 times
As for details of resolution just check out something like Wikipedia and they have info on each of the image suppliers.
http://en.wikipedia.org/wiki/Spot_Image - for the Cnes/Spot Imagery
http://en.wikipedia.org/wiki/Digital_Globe - for Digital Globe
If you want to check out even later imagery then go to the MORE layers, tick Digital Globe and you will see what is available to purchase. You can even just show those images with <10% cloud cover etc. Sinclair Knight Mertz are the agent for digital globe in Aus. Before you rush out for the better photography I suggest you consider the pricing - I got a quote for a friends farm and it was about a quarter of one image and it was over $2000!
Needless to say the current imagery is good enough for him and with Stitchmaps I was able to get a very nice image for next to nothing!
As for Googlemaps - that's something I don't use much but yes, it looks like the same images.