magical core

It is commonly accepted that all lifeforms have a magical core. In this core, some amount of magical energy is stored and when the core reaches a certain size a lifeform becomes capable of using magical energies actively.
So the core of a regular fly trap plant would be small but the core of a magical version would be bigger.

This is, as said above common knowledge.

There are several spells and rituals one can use to find information about one's own core and its properties. From the distribution throughout one's body to fluctuations in the core as well as the rate at which magical energies flow from it or towards it.
Such information can indicate the power that could be used in casting etc.

Show spoiler

The magical core does not really exist.
While active magic users are a pool for magical energies those energies are NOT stored. Not like a cistern or any other form of storing tank.
It simply means that an active magic user is like a wider, deeper part of a river. There simply is more magical energy in that place but it is still flowing.

Magical energy does not flow rapidly in most places. It does not flow like water but more like oil or even honey. At least that is true for most lifeforms. Active users often experience a bit of a faster flow and thus are able to use more magical energy in a short time compared to the average person.
Interestingly the rate of flow can be controlled somewhat by meditation techniques, potions and other means.