Seer is a rare kind of wizard who don't get any spell of his own, has lot of magic energy and high resistance, and ability to see how magic works.
 Voltaire directly said Tedd is seer and  Heka said that's the name for description Pandora gave him ... which was description of Tedd.
It is likely that the "dangerous rarity"
description  Interdimensional Whale used for Tedd was referring to same set of abilities.
Similarly, the rare type of wizard that wouldn't be affected by reset because they don't get spells in the first- the
Disco Wizard from Grace's dream mentioned seems to be this.
Seers are not "chosen" - who is seer is determined by their ancestry.
It may be specific combination of  magic affinities.
Signs of Tedd being Seer Edit
Seeing magic: Despite Sarah assuming that Tedd had programmed a magic vision setting into his glasses, Tedd was actually able to visually sense magic and was surprised that not everyone can.  Tedd first saw this insight using itself, whilst using it on her  mark-based spell when transforming from female to male.
Having log of magic energy: Partially confirmed by Luke's being temporary blinded by his aura.
Being able to disenchant himself: Or herself.
High resistance: He resisted the hair explosion, which surprised him. 
Other Tedd oddities possibly caused by being Seer Edit
He is tested negative by the magic analysis wand.
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