One day I decided to see how many "Now & Then" photos for New York Public Library's Kusakabe prints I could find before running out of energy. Here's tips on how to do the same with an old photo of your choosing.
First step is to find an old photo. It could be in your neighborhood if you like 'em easy. I like a challenge.
Good luck with this one, right? I have no idea where it is, and I know that the transliteration (how to write it in English letters) of Japanese has changed in the last hundred years - I can't trust the caption.
However, the next couple steps involve taking what you know and digging. Keep digging until there is a victor - you or ignorance. Someone knows something about this photo, and they hopefully have posted information on the Internet. All I have is the caption to go on, I know nothing else.
Step two is to dig on the Internet.
If you know of a good archive or history site related to your subject, try there. Presumably there is a reason that the photo was taken, and they may have photos which overlap in purpose.
There are a number of Kusakabe prints on the web, particularly at Nagasaki University. If I get lucky, they have this exact photo with all of the background information including possibly place and date (I'd read it in Japanese to be sure there's no mistake). I'm not lucky this time.
I have a necessary interim step before preceding further: I need to know how to write the subject matter in Japanese. I simply have no clue how to write the names in this print.
I hate Wikipedia as an end point, but it's a good starting point for "presearch". I mosie over to the Japanese site and enter the place name in phonetically (if you've got the fonts, なかのたけ). No dice, the only result is a shrine in the water somewhere - I'm looking for a mountain scene. This usually works, though, because the titles of entries on the Japanese site are written out in hiragana as well.
Step three is is to dig on Flickr itself.
Maybe I should have started here, but for Japanese locations the results are spotty. I definitely recommend it for countries with a large Flickr customer base. In this case, a simple "Nakanotake" query in English letters gave me a splendid, rocky photo. Again, sometimes luck will cause the "now" photo to fall out in the first batch of results.
Looking through the description and keywords I get explanations of the Kusakabe caption. This is Nakanotake Shrine, located at Mount Myogi. I also get the Japanese character names, 中之嶽神社 and 妙義山 respectively.
Step four is to learn about your subject matter.
I go to the web search engines. Luckily the shrine has a website. Their tourist map has 2nd, 3rd and 4th rock gate but oddly not 1st. Now I know how to write 1st Rock Gate in Japanese, though, 第一石門 - it was a literal translation. Other tourist sites for the area reveal the location, which lets me pinpoint it on a map. I don't feel complete until I've spotted the location for any photo.
Step five is to search for your "now" photo on Flickr.
There are very few results, but one is incredibly close to my desired angle. It just needs to be up the slope a bit, but it looks like trees would obstruct the view from there.