How to differentiate between the alters?

Apart from the obvious difference, their personality, there are visible changes between them. This can come in the form of the change in the colour of the iris(es); a mark on their body that only appears when one of the alters are in front, or their flag could change (but that is much rarer). There are a variety of different ways to visibly differentiate them, but due to the lack of information on Hybrids, very few are known.

Sync

It is a state where both the alters are in perfect harmony. It is hard to achieve this; however, it is not impossible, and the hybrids that tend to be in sync more often have a lower chance of Splitting.
A hybrid cannot voluntarily be in Sync, and it can only happen in different circumstances and they tend to be life threatening.
When this happens, each alter can access the thoughts of the other, when usually they cannot, and their overall strength, skill, and awareness of their surroundings combine until they are essentially one person.
However, the downside to this is that it drains their energy, and would be left exhausted afterwards.

Enkaiein (Greek root of the work ink, means 'to burn in')

When a Hybrid approaches the stage of Splitting, their arms and legs would darken to an inky black colour, and would crawl up their limbs until the point when they Split.
If they do not Split, then their limps would remain dark but would slowly return back to normal, but they have a higher chance of Splitting as long they have it. Hybrids tend to cover up their arms and hands as people take advantage of the fact that they could Split, and potentially destabilise a country/kingdom/nation if they succeed.
Enkaiein was first used to describe this condition during the Roman Empire, as the first thing that someone pointed out was its colour, and later on it stuck as its official term. It is also known colloquially as 'Inky Arms' or 'I.A.'.