Finding reactive spells like Overpower or Revenge

[Firesong]

I'm making an addon that will have one button and it will show the correct spell to cast.

So far I have this code working:

local inRange, unit = 0, "target"
if UnitExists("target") and UnitIsVisible("target") and IsUsableSpell("Charge") then
inRange = IsSpellInRange("Charge", "target")
end
if inRange == 1 then
CommandExtra.texture:SetColorTexture(0,255,0)
message = "Charge"
else
CommandExtra.texture:SetColorTexture(255,0,0)
message = "Do nothing"
end

My plan is to have a long-ish if else function for my warrior. My question is how to identify from Lua spells like Overpower or Revenge which flash up on screen and should be used first?

All help appreciated.