How can you tell if an online forum is made with xenForo, phpbb or vbulletin?

[Shelley]

If a forum is made by xenforo it will work flawlessy, if it's vbulletin 5 then it won't.

 

[Strong]

I believe BGG have created it themselves.

It almost looks like a board made from perl!

 

[Kirby]

I see a lot of different forums, some look pretty decent, just wondering, can you tell by just looking at the forum style or source code what software a forum uses? If so, what do you look for?

In many cases it's obvious just by how the GUI does look.
If it is not, I take a look at the page source and search for software specific JavaScript.
If it is till not obious which software is used afterwards it's either an unknown software or extremely customized.

 

[Mouth]

I would have thought most would sites would be showing the software author/owner copyright somewhere. For those that do not, what is the attraction for paying to remove it? Or are they mostly in violation of T&C's?