How to detect an intersection between two lines?

[Brainer]

chronozphere, this doesn't work either. But I think the problem is with these loops:
[pascal]
var
I, J: Integer;
begin
Result := True;

for I := 0 to FPoints.Count -4 do
for J := I + 1 to FPoints.Count -2 do
begin
if LinesCross(TShapeVec(FPoints.Items[I]).X,
TShapeVec(FPoints.Items[I]).Y,
TShapeVec(FPoints.Items[I + 1]).X,
TShapeVec(FPoints.Items[I + 1]).Y,
TShapeVec(FPoints.Items[J]).X,
TShapeVec(FPoints.Items[J]).Y,
TShapeVec(FPoints.Items[J + 1]).X,
TShapeVec(FPoints.Items[J + 1]).Y) then
begin
Result := False;
exit;
end;
end;
[/pascal]

Can anyone tell me what's wrong there? :?