Would work for many using whole genomes. CpG methylation patterns could also work.

"Despite the important role that monozygotic twins have played in genetics research, little is known about their genomic differences. Here we show that monozygotic twins differ on average by 5.2 early developmental mutations and that approximately 15% of monozygotic twins have a substantial number of these early developmental mutations specific to one of them."

https://www.nature.com/articles/s41588-020-00755-1