system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When a baby is born with an extra limb, or the recently craniofacial duplication, how do doctors know if it's a birth defect, or a poorly separated twin or 'absorbed' twin?
gpt
There are a couple of ways to Diagnose this. 

E.g. if its an absorbed twin its fused differently with the rest of the body as it would be if its outgrowth of the body. Often the twin tissue/organs are weirdly connected to the rest of the body and lack certain but normal connections.

Otherwise the cellular make up of the absorbed twin is also likley different, with certain cell clusters being present as residues of the former twin. E.g. stem cell clusters were they should not be, or randomly differentiated cells. Aka the cellular make up of these limbs makes no sense. 

Lastly if they are non-identical twins the genetic make up of the extra tissue will be different.

Though human chimeras (aka a human with two different sets of cells) can go undetected, if the twins cells are seamlessly incorperated into the body. Because in this case its just a regular human.

Funny/tragic anecdote: a british women ones gave birth to a child but wasn't the genetical mother. The father however was. They got in a bit of trouble for some legal reason, until it was discovered that the womens womb was made up of different cells.
As an embryo she absorbed a twin whose cells formed her reproductive system.