[oomp]

Don't use destGUID, use destFlags...

local function GetIconIndex(flags)
    local number, mask, mark
    if bit.band(flags, COMBATLOG_OBJECT_SPECIAL_MASK) ~= 0 then
        for i=1,8 do
            mask = COMBATLOG_OBJECT_RAIDTARGET1 * (2 ^ (i - 1))
            mark = bit.band(flags, mask) == mask
            if mark then number = i break end
        end
    end
    return number
end

This will return a number from 1 to 8 corresponding to each mark, or nil if the target doesn't have a mark, see here for more information. However, if you are intent on using destGUID, I suppose you could do the following, assuming the unit you are checking is a player with a valid unitId:

local function GetRaidTargetIndexFromGUID(guid)
    local name = select(6, GetPlayerInfoByGUID(guid)) -- Get the unit's name; works globally
    local icon = GetRaidTargetIndex(name) -- Returns a number from 1 to 8, or nil
    return icon
end