Even at birth it may be difficult to be sure if twins are identical or non-identical. Identical twins will still be born with their own set of membranes and this is not a marker for them to be non-identical. It’s not always clear simply by trying or examining the twins to know if they're identical or not.
The only way to be 100 percent certain is to do dna testing on each twin and see what genetic info they share. this may be done in the early neonatal period by taking a swab of tissue from inside every baby’s cheek. this can be painless and non-invasive.
The other way to check is to take a biopsy and do blood grouping. Identical twins will share the same genetic info therefore the same genetic markers can be known. Whereas non-identical twins can be very different; a lot of constant as the other siblings with the same parents. They share around half the same genetic info and traits with one another.