Get lighting value of specific point?

Look backwards

Okay, so realistically this is exactly the same as looking forwards, but that's normally against the point. To get the most basic sense of if it is illuminated, you'll have to use a lot of raycasting to get the visibility of lights.

local desc = function(i)
    local queue = {};
    local returns = {};
    local v = i;
    while v do
        local t = v:GetChildren();
        for i = 1, #t do
            local v = t[i]
	    if v:IsA("PointLight") then
                returns[#returns+1] = v;
	    end
            queue[#queue+1] = v;
        end;
        v = queue[#queue];
        queue[#queue] = nil;
    end;
    return returns;
end;

local CachedLights;
local function GetBasicLighting(Point, useImpliedSkyLight)
CachedLights = CachedLights or desc(workspace);
local LightingValue = 0;
if useImpliedSkyLight then
	if not workspace:FindPartOnRay(Ray.new(Point, game.Lighting:GetSunDirection()*500)) then
		LightingValue = 1;
	end;
end;
for i=1,#CachedLights do
	local v = CachedLights[i];
	local p = v.Parent;
	while not (p:IsA("Part") or p == workspace) do p = p.Parent end;
	if p ~= workspace then
		pos = p.Position;
		local dir = pos-Point;
		if dir.magnitude < v.Range then
			if workspace:FindPartOnRay(Ray.new(Point,dir)) == p then
				LightingValue = LightingValue + v.Brightness*(dir.magnitude/v.Range);
			end;
		end;
	end;
end;
return LightingValue;
end;

How does it work?
Really basically. First it checks if it can see the sun. If it can, it assumes it's fully illuminated. After that, it goes through all of the PointLights (And assumes only PointLights) and checks if it can see them, if they're in range, and then adds their brightness as a fraction based on the range.
I'd actually use an RGB value and convert that to brightness later but there's not much difference from a basic standpoint, but if you were being pedantic and really didn't care about performance then that would be the 100% accurate performance-eating lighting value grabber.