What key words should I use in an "assess" question?

The answer to your question would really depend upon what
exactly you are going to assess.  Generally, when you assess something you are
considering its value or worth.  You may be considering how something relates to
something else.  In terms of the study of literature you may be asked to assess the
author's use of particular literary device and how it relates to the meaning of the
work, for example.  In this type of question, you should state examples of the device
and assess what the device does to enhance the experience of the work.  In this case,
assess means to explain.  I don't know that there is a list of words that you should use
when you explain something.  The most important thing to remember when you are analyzing
something to is continually ask yourself why you think something you have said is true,
and then writing the answer to that 'mental' question. 

The
first suggested defintion above was about considering the worth of something -- think
assessment.  In this case, you might be determining if a piece of literature is good, or
even better, what makes it good.  Again, the key to successfully writing this response
is about developing a higher order of reasoning for the points you are making.  You may
state that a story is good because ______________.  That makes it good because _____,
and that is important because _______.  It may look like a lot of becauses, and they
don't necessarily have to be actually in the statements, but good arguments extend the
analysis to prove the validity of the statements made. 

No
matter what you are assessing, focus on proving your point, not on what types of words
you think need to be in your response.  I hope this helps.