Let assume that con artist casts Magic Aura on belt and makes so it looks like 'Belt of Constitution +2'.
PCs don't have Identify - they use only Detect Magic to identify item qualities. Will Detect Magic be able to grant Will save to identify true identity of fake magic item or we need only Identify as stated in spell:
http://www.d20pfsrd.com/magic/all-spells/m/magic-aura