For example: what's webhosting digital forum used? how can we check it? I have tried to using some service like whois, but I only get the domain info.
You could find out host of any website using its IP. You will get details info of that ip. Apart from this, you can try Who Is Hosting This?
It's a process, you first need to gain the ip address that is assigned to that domain, from there you do a whois on the ip address and there is all your information.
It will probably not be useful if you will try this method for digitalpoint or another project which use their own servers. If the website uses shared hosting, the root server is very often named like something.company-name.com.
Yep, you're right Nice share Jose, actually this is the method that I run now yes, just work with shared hosting