I've used Google a few times to date negatives and photos. First was in trying to identify which great uncle appeared in an old photo I was restoring. Thankfully, he was leaning against his truck and the licence plate was clearly visible. I'm not sure about elsewhere, but here in Victoria, Australia, the numbering system changed through the decades and, later, the "catch phrase" above the numbers and letters changed. That gave me a ballpark figure and, given the twenty year age gap between the youngest and oldest child and a known date of immigration from Ireland to Australia, I had my year and the correct family member.
The second was while developing old film for a friend who sells antique cameras and hands over any film he comes across sitting inside them. It's been hit and miss on visible results but, those few that show an image have often included street signs and zooming down to street view in Google Maps gives me not only an exact location, but a "then and now" comparison of the street which I love. Again, the licence plates have helped me attach a date to those negatives but with nowhere near the accuracy Bill's obtained as these weren't my negatives.
It sure beats hunting through historical books trying to date them by the people's clothing!
The third, and my favourite discovery, was of an old photo of a great uncle aboard what my aunt thought was the ship on which he immigrated to Australia. Cleaning up the image revealed the life belt hanging between the railings which gave me the "ship's" name and a Google search told me it was actually a paddle steamer which took passengers from the capital city down to a seaside suburb half an hour away from where I now live. Amongst the archived footage kept by the Australian Broadcasting Commission was a short film showing passengers boarding the paddle steamer and arriving down in Sorrento, disembarking to be taken by horse drawn carriage into the town. While I couldn't spot my great uncle in the crowds, it still gave me a cheerful goofy grin to find moving imagery from that time.
Shop windows/businesses are also a way to get some idea of when a photo was taken if they were there long enough to establish any sort of reputation. I put together a family tree for a group of photographs purchased by the abovementioned antique dealer and my starting point was a couple of first names scribbled on the back of one of the photos, a shop window with the town included in its name and another photo of one of the boys photographed in his army uniform. Being a small town, when he was killed in action, his name featured prominently in the local paper at the time. From there I managed to put names to every member of the family in the photographs from obituaries, wedding announcements, births, etc.
Apologies for waffling on, but I love old photos and their stories