How to identify the twins are identical Scientifically?

DNA test is the most accurate of all. Other methods are Blood test or Examination of Placenta. but these are less accurate as compared to DNA.

All the identical twins has to be same-sex i.e., Boy/Boy or Girl/Girl. Until very recently, we know that there are certain rare instances when identical twins can actually be different-sex individuals; when this has occurred and been documented, the twins are genetically identical in every way other than a slight chromosomal difference.
But make no mistake, not every set of boy/boy or girl/girl twins is identical. In fact, many same-sex sets of twins are fraternal twins. Very often, same-sex twins look very different, indeed, sometimes right from birth. They all have different-color hair and eyes, are of different height and have different physical builds, and display very different mannerisms.
When it comes to triplets, quads, quints, and even larger higher-order multiples, they are the combination of one pair of identical & one pair of fraternal set.

(*Source Twinsmagazine.com)