Good question! Christopher Sly is a commoner, unpolished and drunk to say the least. The Lord and the hunting party that come upon him are more polished, educated, and come from noble blood. In Shakespeare's plays, the commoners (servants, peasants, etc.) are depicted by prose--non-poetic language which indicates their lowly status in society. The noblemen (Lords, Ladies, Kings, Queens, etc.) all speak in poetry in Shakespeare's plays in order to depict their place on society's ladder. In Shakespeare's time, poetry was considered to be the more elevated and sophisticated writing genre. Prose was thought to be less challenging and easier to write, so therefore it didn't get the recognition that poetry received (in fact, prose wasn't really recognized until the about 1800's when the novel came to be).
Therefore, Christopher Sly is going to speak in less eloquent prose language, and his "buddies" will be speaking in poetry. In addition, the whole joke the Lord and his friends are putting over on Sly makes the entire beginning of the play comical. They have decided to dress him as a Lord and to make him believe that he holds that status as they watch the play together. Sly's commentary on the play and on the situations in the play show his rough edges, along with the words he chooses...all of which illustrate that he is most certainly not a gentleman of noble blood, no matter how much his companions swear that he is one of "them".
Go back now, and look for differences in their speech patterns. Don't forget to check the footnotes in your text that will indicate which words would be considered vulgar or undignified. Once you train your eye, you will easily recognize the difference between the educated nobles and the uneducated, unpolished Sly.
You will have to focus on the first part of the play, however. Sly disappears and he isn't mentioned again even though it's supposed to be a "play within a play" with Sly and his companions watching from the audience. Some critics say that Shakespeare just forgot about him. While I don't think Shakespeare is that careless, I don't have another explanation as to why all references to Sly suddenly drop from the play for good. Maybe this is a good question for your classroom teacher. Good Luck!