Implantation bleeding is the amount of blood outflow that occurs when the blastocyst settles into the uterine walls, causing the rupture of many small capillaries which build up the wall, in order to create a connection with the circulatory system of the mother. 
While many wonder about the validity of the occurrence of implantation bleeding, other claim it is true. While some doctors shove this away as a rumor alongside that of the mother being able to feel the implantation, others normally discourage reliance on bleeding due to implantation as a positive test for pregnancy because the lines between that and other types of bleeding can often get blurred. One of the reasons as to why this is so, is because, usually, it is also not practically feasible to conduct pathological studies on the sample of blood because of the medical complexities involved. Also, many doctors also claim that implantation bleeding occurs only exceptionally and in occasional cases, thereby denying it the status of the rule of the thumb.
However, for those who wish to go through a step-by-step process of barrier breaking when confirming their pregnancy, looking out for implantation bleeding will only require you to know what it looks like.
The bleeding that occurs due to implantation is generally diversified from pink to brown in color. Having a pink or red outflow, will determine that the patient is having an active site from which the blood has produced - indicating implantation has taken place. Having a brownish outflow will indicate that the implantation into the uterine walls has occurred some while back.
The particular color of the bleeding will also determine the amount of blood that shows up on the panty. Recognizing what does implantation bleeding look like, will be further augmented by knowing what they look like in terms of amount. For those experiencing a pinkish or reddish outflow of implantation bleeding, the amount might be slightly more than those experiencing the brownish outflow. This is because, since they are more in quantity, they take a relatively less amount of time to reach the cervical opening. On the other hand, the brownish stains will be less in quantity, which explains the color because, the lesser the amount, the more the time it will take to reach the cervical opening. 
Still sounds like any other regular menstrual discharge? Well, there is another factor that defines what does implantation bleeding look like. All discharges that occur during bleeding due to implantation, lack the characteristic flowing quality of menstrual discharge. All discharges related to implantation bleeding will appear as a smear, a stain, or a light discharge on the panties. If the discharge possesses a flowing quality, this might indicate that menstruation has begun, or some harm has occurred to the pregnancy, thereby calling for immediate medical help.
However one should be warned, that these very same physical characteristics are possessed by many a bleeding caused by other reasons such as stress, pre-menstrual discharge, miscarriage discharges. Therefore, even if the patient knows what does implantation bleeding look like and can recognize it, it is better to also take some more solid tests.