You can remember this by heart but you can also derive this.
if $$y = x^2 + 1$$ then $$\frac{dy}{dx} = 2x$$ which shows $$\frac{dy}{dx} \geq 0$$ if $$x \geq 0$$ and $$\frac{dy}{dx} \leq 0$$ if $$x \leq 0$$
Hence the curve first goes down and then up which makes it a cup form.
Similarly
if $$y = -x^2 + 1$$ then $$\frac{dy}{dx} = -2x$$ which shows $$\frac{dy}{dx} \leq 0$$ if $$x \geq 0$$ and $$\frac{dy}{dx} \geq 0$$ if $$x \leq 0$$
Hence the curve first goes up and then down which makes it a cap form.
Reinout 
