Short Answer 
To determine the origin of a letter without a return address, first inspect the postmark for clues about its mailing location. However, keep in mind that postmarks only indicate where the letter was dropped off, not the sender’s actual address, and may have limitations like being unclear or missing. Combining postmark information with other details from the letter can aid in better identifying its source.
Step 1: Observe the Return Address
If a letter comes without a return address, your first step is to search for any clues that might indicate where it was sent from. Focus mainly on the postmark, which is a special stamp applied by the postal service. This postmark typically includes the date and the originating postal location, helping narrow down its point of origin.
Step 2: Analyze the Postmark
Once you’ve located the postmark, take a closer look at it. Keep in mind that the postmark only shows where the letter was mailed, not the sender’s personal address. For instance, if a traveler sends a letter from a vacation destination, the postmark will reflect that location. Be cautious, as postmarks can sometimes be difficult to read or may even be missing altogether.
Step 3: Consider Limitations of the Postmark
While postmarks can be informative, they are not foolproof indicators of the sender’s true location. Always be aware of the limitations, such as instances where letters are mailed from automated machines, which may produce unclear postmarks. Thus, it’s essential to combine the information from the postmark with any other details you may find on the letter itself for a more accurate identification.