All the route helpers names can be find inside Rails.application.routes, like this:

Rails.application.routes.named_routes.helper_names
It'also possible to get the path helpers only, or the url helpers only:

# path_helpers
Rails.application.routes.named_routes.path_helpers_module.instance_methods
# url_helpers
Rails.application.routes.named_routes.url_helpers_module.instance_methods
We came up with these solutions, but if anybody knows better let us know!

Sources:
Stackoverflow - In Rails, how to see all the “path” and “url” methods added by Rails's routing? (update: using Rails console)