I think that there are several approaches to take when evaluating any work. If you have been given guidelines from instructor that need to be followed, I would adhere to those more than anything else. I do believe that one element of evaluation that should be used in assessing Chekhov's work is how he has used the situation in the drama to reflect a social statement. In many ways, the predicament outlined in the work is representative of how people deal with change and adaptation to contingent circumstances. A manner of evaluation for this drama is to examine how Chekhov brings out the failure of individuals to understand the force of social change and what forces bring about this level of change. The response of Mrs. Ranevsky can be seen as representing a sense of nostalgia or an affinity for the way things were, but what makes the play so compelling is that her character ends up representing what it means to be on the opposite side of social change. In the same manner, I think that assessing the attributes and detriments of Lopakhin as a force of social change would be another way to evaluate the work in providing a paradigm as to how social change can be understood.
If I were you I would start by saying whether you liked the play or didn't like it. Then you might tell what you liked and what you disliked, and why. Your instructor probably wants your own personal reaction more than anything else.