[Caellian]

Not sure i understand what you meant with the function GetSpellName but here's what i did, if that's correct.

local _, caelUI = ...

caelUI.readycheck = caelUI.createModule("ReadyCheck")

local GetSpellName = function(id)
	return GetSpellInfo(id)
end

local flasks = {
	-- Cataclysm
	GetSpellName(80719),		-- Flask of Steelskin
	GetSpellName(80720),		-- Flask of the Draconic Mind
	GetSpellName(80721),		-- Flask of the Winds
	GetSpellName(80723),		-- Flask of Titanic Strength
	GetSpellName(94162),		-- Flask of Flowing Water

	-- Pandaria
	GetSpellName(105694),	-- Flask of the Earth
	GetSpellName(105691),	-- Flask of the Warm Sun
	GetSpellName(105696),	-- Flask of Winter's Bite
	GetSpellName(105693),	-- Flask of Falling Leaves
	GetSpellName(105689),	-- Flask of Spring Blossoms

	GetSpellName(127230),	-- For testing purpose: Vision of Insanity
}

ReadyCheckListenerFrame:SetScript("OnShow", nil)

local isGuildGroup

caelUI.readycheck:SetScript("OnEvent", function(self, event)
	if event == "PLAYER_ENTERING_WORLD" then
		hooksecurefunc(GuildInstanceDifficulty, "Show", function()
			isGuildGroup = true
		end)

		hooksecurefunc(GuildInstanceDifficulty, "Hide", function()
			isGuildGroup = false
		end)
	elseif event == "READY_CHECK" then
		PlaySoundFile([[Sound\Interface\ReadyCheck.wav]], "Master")

		local food = UnitBuff("player", GetSpellName(104280))

		local flask

		for i = 1, #flasks do
			flask = UnitBuff("player", flasks[i])
		end

		if (not isGuildGroup and food) or (isGuildGroup and flask and food) then
			ReadyCheckFrame:Hide()
			ConfirmReadyCheck(1)
		end
	end
end)

for _, event in next, {
	"PLAYER_ENTERING_WORLD",
	"READY_CHECK",
} do
	caelUI.readycheck:RegisterEvent(event)
end

But now i get:

Error occured in: Global
Message:
..\AddOns\caelUI\modules\misc\readyCheck.lua line 52:
   Usage: UnitBuff("unit", [index] or ["name", "rank"][, "filter"])
Debug:
   [C]: ?
   [C]: UnitBuff()
   caelUI\modules\misc\readyCheck.lua:52:
      caelUI\modules\misc\readyCheck.lua:35