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Ways of determining zygosity

"Of course my twins are not identical-their socks are different colours"

There certainly can be much confusion in the early years, as DZ twins may look and behave as similarly as MZ twins. But by the age of 3-4, then questionnaires such as this one can help. The 'gold standard" is of course genetic testing, done in the past by bloodtyping but increasingly by directly examining the DNA. Previously a blood sample was needed and many medical staff were unwilling to take blood from young children, just to resolve uncertainty over zygosity. Now DNA can usually be obtained successfully from cells gathered by brushing inside the cheek and more and more laboratories are offering DNA zygosity testing. It is still expensive and for many families, completing a questionnaire will resolve things in their minds.