One of Ves' innate skills, [Life force Detection] is a passive skill unique to the zombie side of her. And just as the name implies and as said before, it's a skill that allows her to detect life force in a 50 meters range, even obstacles like walls or other mundane things blocking her sight.
In her sight, the life force of all lifeform is like a bright burning green flame and using the intensity at which it's burning, she can deduce the level of lifeform.
Though it's mostly accurate but not always. She has tried on Leo and the intensity at which his life force was burning didn't correlate to his level.
To give an analogy, the life force of other people, zombies and monsters she has seen were no bigger than a candle flicker.
The biggest she has seen is the size of a softball and that belonged to the level 9 Mutant Boar. But Leo's life force was way bigger and it burns brighter than other, not only that, it was colored strangely.
