Question is: How would the PCs be able to get information on whether a particular dragon casts spells as a sorcerer or wizard?
First, as D'karr said, there is no mechanical difference between the manner in which a sorcerer casts spells and a wizard casts spells. Both are subject to the same Spellcraft checks and components rules, can be counterspelled in the same manner, etc. Unless there were living witnesses who could testify as to whether the dragon used spellbooks, it does not appear that a detectable difference in casting ability would exist.
Second, how would anyone have this sort of information on a particular dragon? Dragons are relatively secretive creatures who lair in remote areas far from the purview of humanity. The only individuals likely to have studied a particular dragon's spellcasting habits are adventurers who had engaged the dragon in combat; if the dragon is still living, it is probable that those adventurers are not. Pretty much the only means of identifying a particular dragon as a "wizard spellcaster" vs. a "sorcerer spellcaster" would be that suggested by Voadam (wizards and dragons trading spells); a situation that is unlikely at best.
The "precedent" mentioned by hong is that set forth in the description of the white dragon Arauthator on the WotC website. Arauthator's sorcerer levels (Sor5) stack with his innate spellcasting ability.
Wiz/sor is a relatively inefficient multiclass for PCs; on an NPC, where the levels can be added willy-nilly, it's potentially quite powerful, since you can combine the flexibility of casting a few known spells using spontaneous casting with the vast array of spells known as a wizard. Also, the bonus metamagic feats from wizard levels are even more useful in casting sorcerer spells, as a rule.