When talking about fast food, I guess you have to split into two categories: Those restaurants operated by (inter-)national chains and "Street food". Why?
When going to an unknown country, most people feel uneasy about eating just at the first place they come across due to hygiene considerations. For example, Starbucks in China is the prime place to go when you want to make sure that the food is safe and the toilets are clean if - specially in more remote areas of the country - you do not have the time to search for a reputable location. However, what you normally get in those places is not local food. To operate large, national chains one needs often a strong brand that can draw in the masses you need to justify the sophisticated routines you can see behind a counter of a McDonalds or KFC. That is not to say that there are no non-american fast-food chains. There are Pho Noodle chains in Vietnam, Ramen chains in Japan etc. However, for the added safety bonus, you will limit yourself quickly to larger countries where such an operation makes sense and there also to largely US companies with 1-2 exceptions of local franchises, depending on the size and development status of the country. Further, you will also limit yourself to a much more narrow selection of food, since each of these restaurants will offer the same menu nationwide. Maybe companies like McDonalds serve a slightly changed menu in Japan, but within Japan, the dishes are mostly the same.
So in order to get a truly local experience with the varieties each region or even city can offer, I would recommend to use the term street food instead of fast food. And there, you will find very quickly a huge variety of web sites, lists and directories online that will fill your every wish - but require you to keep your eyes open in terms of hygiene of the food. Since street food vendors often do not have bathrooms, that issue solves itself.
And that search in return would allow you to find out what food types are popular for each area of a country (let's say Ramen in Japan), and you can then go back to google and search for example "Japanese Ramen Chain" to find something with a more consistent quality.
So some good lists for street food are Wikipedia and even restaurant guides. Also Anthony Bourdain from the travel channel network is usually having some kind of street food during his visit to each country. You might even take a look at the webpage of the World Street Food Congress. Flickr is having a group that allows you a more visual experience to explore. And last but not least, googling for street food is a valid option, too.
