How can you tell which form factor a computer case is from the outside? ATX or BTX?

What is they physical differences between ATX and BTX cases from looking at it from the outside?How can you tell which form factor a computer case is from the outside? ATX or BTX?
I'm not sure you can tell just by looking at the outside. If it's a Dell or Gateway it's probably a BTX.





These might help:


http://en.wikipedia.org/wiki/BTX_(form_f鈥?/a>


http://en.wikipedia.org/wiki/ATX