Is there a difference between endometrial tissue that exactly looks like gestational sac and the real gestational sac? or only the lab test can tell?

Lab test...: A gross and microscopic examination of the tissue by a pathologist is the best way to detect a gestational sac in a miscarriage tissue specimen. A pathologist can sometimes tell by looking at the tissue if there are placental tissues present, but oftentimes fragmented endometrium can have the same appearance.

Only the lab : Generally only the pathology lab can tell the difference.