This even works on written forms of summoning spells. Incidentally, it can also detect beings summoned by spells, just so long as the spell is actively keeping them around or is otherwise active.

Detect (Summoning Spells, Rare) [17] 
    Based on Activation Roll [+5%] 
    Precise [+100%] 
    Analyzing [+100%] 
    Cosmic (May disregard undesired summoning spells) [+50%] 
    Magical [-10%] 
    Spell Components [-20%] 
        Must say "summon" in Latin 
        Must gesture with hands 
Level 2 [18] 
    Add Extended Duration (x3) [+20%] 
Levels 3-7 [19/20/21/22/23] 
    Increase Extended Duration by 1 step [+20%]