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.

We will check with the client requirement

Testing GUI Application :

GUI software testing is the process of testing a product
that uses a graphical user interface, to ensure it meets its
written specifications. This is normally done through the
use of a variety of test cases.
Most clients in client/server and web-based systems deliver
system functionality using a GUI. When testing complete
systems, the tester must grapple with the additional
functionality provided by the GUI. GUIs make testing systems
more difficult for many reasons: the event-driven nature of
GUIs, unsolicited events, many ways in/many ways out and the
infinite input domain problems make it likely that the
programmer has introduced errors because he could not test
every path.

Available literature on testing GUIs almost exclusively
tends to focus on tools as the solution to the GUI testing
problem. With few exceptions, papers on this topic pay
attention to GUI test design, but have concentrated on how
automated regression-test suites can be built and
maintained. One seminal paper on the subject formulates a
GUI test strategy to detect errors and use tools to assist
this process