Simple question... can detect magic detect the Change Shape supernatural ability?
The Bestiary says Change Shape functions as a polymorph spell (or whatever spell is listed in the creature's description), but supernatural abilities aren't subject to all the limitations of spells (spell resistance, counterspells, or dispel magic), and they don't function in antimagic areas.
Supernatural abilities are magical, and Detect Magic detects magic. All of the various Change Shapes function as one of the polymorph spells. So I would say yes, Detect Magic works against Change Shape.
The detect magic table indicates spells and magic items. I would assume spell-like abilities get detected as spells, but I believe that supernatural abilities are not detected. I am made especially sure of this by Arcane Sight and its mention of spells and spell-like abilities but not supernatural abilities.
This is one of those cases where "functions as" is not as clear as one would like. I think the best interpretation is that the Polymorph spell dictates which forms can be taken (limited further by the Change Shape quality), and which abilities come with it (limited further by Change Shape: no ability score change), and perhaps casting time (it doesn't say), but "functions as" doesn't also bring in magic attributes like caster level, school, and subschool.
... and perhaps casting time (it doesn't say) ...
As far as casting time goes, "Use supernatural ability" is listed in the list of Standard Actions in the Actions in Combat table:
Supernatural Abilities (Su): Using a supernatural ability is usually a standard action (unless defined otherwise by the ability's description). Its use cannot be disrupted, does not require concentration, and does not provoke attacks of opportunity.