There is not a DNA-like test for brain cells. They are identified based on their shape and behavior. Testing them genetically to determine that they are brain cells is complicated. Furthermore, extracting the genetic material destroys the cells—which defeats the purpose if you are trying to use such tests to determine “personhood.”
In a tiny embryo that is just a few weeks old, you can see the brain. And since it is undeniably there, you can be certain that it is doing brain functions, meaning that the embryo has a rudimentary sense of awareness.
In order to try to discredit the fact that an organism with a brain is aware and feeling, the abortion industry has come up with the notion that the brain is somehow inactive until some event magically turns it on. This is completely untrue, and would be laughable if not for the fact that this lie is meant to promote abortion. Every kind of cell functions according to its cell type, including brain cells. There are no magic switches that keep cells turned off until some arbitrary point. When cells do not behave according to their type, it is because they are dead—and that condition is irreversible.
I call this the liberal's "Poof Theory" of when a preborn person becomes human. Under this theory, when a woman decides she is going to keep her baby and not kill it via abortion, a signal moves out of her brain, moves down the spinal column, crosses over into the umbilical cord, and eventually makes it way to the preborn person, then POOF!, it's a person. Amazingly, this theory holds up in court.