For any testing there should be some set of standards to be 
followed.Particularly in GUI testing,look and feel should 
be good.
We should follow the requirements specification documents 
for GUI testing.
There should be some screen shots (given by client) which 
we should follow as it is.
And for button sizes, font,font size ,colours used,placing 
of links,objects and the placing of the objects in the page 
should be followed some standards.
If we take a button in the page that should be some 
standard size. if the size of that button is more or less 
the client feel bad about that.
So we should have miimum common sence while tesing GUI 
testing.
some time there may be some mistakes in the screen shots 
provided by the client also,but that is our responsibility 
to raise those issues.