Average "mutt" unicorns and some other ungulates possess magic but their bodies are not saturated with it, their magic is solely in their horn(s) or near them.

Purebloods' bodies are fully saturated with magic, every part of them has magic and is influenced by that magic-- magic for them is like blood and other important fluids, it's essential for their functioning.

They have special organs scattered around their body (think of them as being akin to lymph nodes in that regard) that produce magic as well as help them process the slim pickings of magic they absorb from the atmosphere.

The magic organs vary from individual to individual, most have average organs which give them average magic levels, some have strong organs and some have weak, faulty ones.
Abnormally high or low levels have stigmas associated with them that aren't without reason...

The reflection, coloring, pooling, and splatter (though small) all look really good

Abnormally high or low magic levels are quite uncommon, /very/ high or low levels are rare because the foals often die. Low level foals tend to die from the stress low magic puts on their growing bodies and high levels just occur less than low levels.

Low level Purebloods have a particularly tougher time than any other magic level, the low magic level affects every bit of their body-- everything from the immune system, muscular structure, ability to process and endure things, to eyesight, hearing and fertility. Hearing takes the least of a hit while fertility takes the worst.

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A Pureblood's magic level/vitality can be assessed in a few ways, one of the most reliable ways is to check their blood, as magic concentrates in it a bit more than anything else. (Tissues and organs show magic
strongly as a result of this)

A Pureblood's ability to use magic for things like spells and simple manipulation (levitation) is also an indicator but not a really reliable one as some may just have trouble with using magic in that way. (Often in childhood)

The defining characteristic of those with high magic levels is eyes that constantly glow, this is something they can't hide (unless they cover their eyes) so anyone strong stands out whether they want to or not...

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Finally, since magic saturates the body so much, blood (as has bern said) and any bodily fluids will show the Pureblood's magic level. Blood has an iridescent sheen that matches the individuals magic level, the stronger the sheen/visible magic color the stronger the magic.
Other things like tears will glow faintly with the color of the magic, higher levels glowing more brightly.

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(That's it I think.
I'm tired lol)

Also: Magic color always matches eye color.

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okay /that's/ it

mmm tasty syrup