There's no good source on back end tech market share. The simple truth is that there's no way to know what a server is running unless the server lets you know.
I'd say instead that there's no guaranteed way. There are likely markers though. For example if you navigate the website and get a cookie set with name "..._laravel", you can safely assume it's a PHP app for stats purposes. There are also more subtle things like the format of csrf, format of error responses, behaviour for weird data passed in (especially name[]=value), etc.
PHPSESSID :)
only if the site started a session
Indeed, but that's not really a good source to estimate market share as there will be a strong bias towards stacks that are more conspicuous.