Surface touching something?

I need to find out if the surface of a certain part is touching a part. For example, if the part's bottom surface is touching a different part, it would return true, otherwise false. However if that same part's top surface was touching a part that would not matter, it would only see if the bottom surface of a part is touching something.

Move it

This assumes that your part isn't actually intersecting with another part, and that you don't need the part which it's touching.

local function IsTouchingForward(p, closeness)
  local closeness = closeness or 0.1;
  local tmp = p.CFrame;
  local tmpp = (tmp*Vector3.new(0,0,-closeness);
  p.Position = tmpp;
  local r = p.Position ~= tmpp;
  p.CFrame = tmp;
  return r;
end;

This checks if there's something which it can collide with 0.1 studs infront of it. You can change the vector and closeness to tune it to suit.

This is a fun question!

Another possibility is to do some plane intersections along with culling. The calculation is way more intense, but the answer is more accurate and allows you to test specific parts. I'd still rcmd using eLunate's tho if it doesn't matter.

function planeIntersect(point, vector, origin, normal)
	-- see this blog post: https://scriptinghelpers.org/blog/silhouettes-and-shadows
	local rpoint = point - origin;
	local t = -rpoint:Dot(normal) / vector:Dot(normal);
	return point + t * vector, t;
end;

function surfaceCollision(part, normalid, otherpart)
	local planes, corners = {}, {};
	-- get the planes that make up the surfaces
	for _, enum in next, Enum.NormalId:GetEnumItems() do
		local lnormal = Vector3.FromNormalId(enum);
		local wnormal = part.CFrame:vectorToWorldSpace(lnormal);
		local distance = (lnormal * part.Size/2).magnitude;
		local point = part.CFrame.p + wnormal * distance;
		planes[enum] = {
			normal = wnormal;
			point = point;
		};
	end;
	-- get corners of thing you're checking
	for x = -1, 1, 2 do
		for y = -1, 1, 2 do
			for z = -1, 1, 2 do
				table.insert(corners, (otherpart.CFrame * CFrame.new(otherpart.Size/2 * Vector3.new(x, y, z))).p);
			end;
		end;
	end;
	-- collision lines
	local attach = {
		[corners[1]] = {corners[3], corners[2], corners[5]};
		[corners[4]] = {corners[3], corners[2], corners[8]};
		[corners[6]] = {corners[5], corners[2], corners[8]};
		[corners[7]] = {corners[3], corners[8], corners[5]};
	};
	-- check each line against the surface plane chosen
	for point, set in next, attach do
		for _, corner in next, set do
			local v = corner - point;
			local p, t = planeIntersect(point, v, planes[normalid].point, planes[normalid].normal);
			if t <= 1.001 and t  >= 0 then -- minor margin of error to account for float math error
				local pass = true;
				for key, plane in next, planes do
					if key ~= normalid then
						local r = point - plane.point;
						if r:Dot(plane.normal) >= 0 then
							pass = false;
						end;
					end;
				end;
				if pass then return true; end;
			end;
		end;
	end;
	return false;
end;

Example in action: http://i.imgur.com/6aeVvMe.gif