Twin Zygosity Test
One way to determine whether a set of twins is identical or fraternal is to test their DNA. The test, also known as a 'Twin Zygosity Test' requires a swab sample from each twin. This sample is then sent away to a lab to be tested and the results which come back determine what type of twins the two infants, children and even adults are.
It is far more common for a set of twins to be dizygotic, accounting for 28 out of 1000 births in the United States, whereas monozygotic twins only account for 3 of these 1000 births. This means that the majority of samples which go away and are tested at the lab come back with the verdict that the patients came from two separate fertilized eggs but for many different reasons some twins and parents wish to know if a set of twins is otherwise monozygotic.
The majority of these tests are carried out in order simply to answer the question and give peace of mind. After all, I'm sure you would like to know if you shared almost 100% of your DNA with your sibling or not. Other reasons for the test include preparation for possible future health complications; identical twins are perfect donors for blood, bone marrow and even organs if their sibling were to fall sick. Some twins or the parents of twins also require tests for legal reasons (i.e. custody or immigration).
The actual test results usually take just under a week to be returned from the lab and are very reliable. The twin zygosity test itself simply involves a sterile cotton swab (otherwise known as 'Dacron synthetic' swabs) being rubbed against the inside of the cheek (two for each twin and each side of their cheeks, left and right) to collect cells. These swabs then need to be packed up securely in swab storage envelopes and sent off to the lab for testing. Some agencies will allow you to send off your own swabs if the test is simply to satisfy curiosity but in legal cases the swabs need to be obtained by a medical professional such as a doctor or nurse.
This remarkably simple and non-intrusive test can give astounding results. Some twins might find that when they thought they were dizygotic from their individual differences for years, they actual share a lot more DNA than they thought because they are in fact monozygotic and came from the same fertilized cell. One of the best things about this kind of testing is that it can be carried out at home and the results can be sent back within a week so there are no hospital visits and the whole process is very private.