You can read the enhanced version of the Prologue here on eNotes. What you will find are setting markers that place it in the 14th century. First of all, a group of people is making a pilgrimage to the shrine of Thomas a Becket at Canterbury. Pilgrimages to this shrine were very common at this time period. Chaucer's group is leaving from Southwark to go to Canterbury. They are meeting at an Inn, the only form of lodging in this time period, and they are walking and/or going on horseback, so you know it is not modern times. There were many inns and taverns in Southwark at this time and it was a popular starting point for pilgrimages.
At the end of the Prologue, the author says that he is going to describe the pilgrims and that he will start with a knight. Again, this is a clue to the time period.
Also, the Old English language in which the Prologue is written is a clue to the time period.