Day Seventy-five of Twinipedia:
Fact #75
Zygosity testing aka "how many eggs where in there to start with testing?"
Are those Twins identical or fraternal? It is impossible to tell just by appearance. Plus, some identical twins may be born with individual sets of membranes, which may lead to the mistaken assumption that the babies are fraternal. 
One way to tell the difference is to have the twins DNA-tested. Identical twins share the same genetic information, while fraternal twins share about 50% of their genetics. The test can be done with a sample of cheek cells, collected without any needles or blood draws. Although there are other testing types that include blood group examinations.
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