Which Destruction spells benefit from the "Impact" (Dual Cast Stagger) perk?

The Destruction skill tree's "Impact" perk (req: 40 skill) has the following description:

Most Destruction spells will stagger an opponent when dual cast.

Which spells qualify?

The strategy guide confirms your findings: the only spells not affected by it are Flames, Sparks, and Frostbite. Every other dual-cast spell will cause stagger.

However, after some testing and checking with the UESP, it's not accurate. Here's what I've found is benefited by Impact:

• Apprentice-level direct damage: Firebolt, Ice Spike, and Lightning Bolt
• Adept-level direct damage: Fireball, Ice Storm, Chain Lightning
• Expert-level direct damage: Incinerate, Icy Spear, and Thunderbolt

Things that are not affected by Impact:

• Novice-level direct damage: Flames, Frostbite, and Sparks
• Master-level area-of-effect spells: Fire Storm, Blizzard, Lightning Storm
• Cloaks: Flame Cloak, Frost Cloak, and Lightning Cloak
• Runes¹: Fire Rune, Ice Rune, and Lightning Rune
• Specials: Arniel's Convection, Vampiric Drain (can't be dual-casted anyway)
• Walls: Wall of Flames, Wall of Frost, Wall of Storms

So, 9 spells affected, 17 that aren't. Definitely not "most" Destruction spells even if you included Runes.

Notes

¹Runes have an inherent knock-back effect: they could be affected by Impact, but it doesn't seem to make a difference either way.