How would one find out if it's even possible on specific site?
For example https://forums.eveonline.com/default.aspx?g=topics&f=257
There are many more sites where I wanted to display more results per page but option is not available.
Without knowledge of the code base, there is no way to know whether you can change the page behavior via a URL parameter, other than trial and error.
If the site is well designed, all URL parameters ought to be validated against a white list, so it should not be possible to hack the URL. So you should not rely on this.