that is, to tell the current Rails version it should use, and also to find out what version of Rails started this project.
For Rails 2 you can go to
config/environment.rb and look for
RAILS_GEM_VERSION definition at the top.
For Rails 3 go to Gemfile and look for
gem 'rails'... at the top.
These tells you what version it’s using now. But doesn’t mean it was created with the same version and there’s no way to be sure. It might have been upgraded to a newer version.