Thanks to next-generation sequencing, a trusty (crusty?) old plot device is now much less credible. This BBC article describes how investigators are looking at the mutations between twins over the entire genome, not just in the million or so SNPs that are typically tested. With on the order of a couple dozen variants between identical twins, you’re got a fairly good chance of telling the twins apart, especially if you have sequences from their relatives that show similar mutations.
But really, we all know it’s the one with the goatee, right?