Contrast with Arcane Sight, which lets you determine the existence of SLAs which aren't in use...
If you concentrate on a specific creature within 120 feet of you as a standard action, you can determine whether it has any spellcasting or spell-like abilities, whether these are arcane or divine (spell-like abilities register as arcane), and the strength of the most powerful spell or spell-like ability the creature currently has available for use.
So yeah - Detect Magic, which doesn't mention anything like this, won't let you detect inactive SLAs.