Could doctor know if any malignant cells has been left behind after surgery? what if the prioximal and margin are clear?

Possibly: Your surgeon evaluates the margins of your specimen under the microscope with a pathologist at the time of the operation. This is done as a "frozen section" of the specimen. Later, the pathologist evaluates the specimen with permanent stains and markers. This is the final determination of the specimen and its characteristics. Your surgeon should be able to tell you the specifics of your specimen.

Size and location: Your surgeon could possibly know this information, depending on the size and location of the tumor. Your surgeon may ask the pathologist to assess whether there are malignant cells at the margins during surgery (frozen section), or he may wait for the final results which can take a week or longer. The larger the tumor is and the wider the area it encompasses, the more difficult this will be.