When doctor does internal exam they can touch the cervix which is the mouth of the uterus. The cervix during pregnancy is long thick and tightly closed. Closer to delivery it becomes shorter and softer. Hence doctor can predict that delivery will happen soon.

Mam my bhabhi told me that by the end of 8th month doctor told her without sonography that delivery can be anytime in 15 days..how is it possible to tell?...my doctor said she will start p/v before one month of due date

Pelvic examination is generally done close to full term. This is to assess the cervix. Prior to that it is not required.