Can we guess the periodic/aperiodic nature of motion from the equation of motion?

A good way to look at these sorts of problems is to talk about conserved quantities. For instance, with some manipulation, it is easy to show that something like energy is conserved in the first formula: $$\ddot{\theta}+\frac{g}{l}\sin\theta=0=\ddot{\theta}\dot{\theta}+\frac{g}{l}\dot{\theta}\sin\theta=\frac{d}{dt}\left(\frac{\dot{\theta}^2}{2}-\frac{g}{l}\cos\theta\right)$$

And so the quantity $\frac{\dot{\theta}^2}{2}-\frac{g}{l}\cos\theta$ is conserved. Since the angular velocity is only a function of position, we are left with three possibilities:

1. The motion is unbounded: goes off to infinity. This is not possible here because $\theta$ is periodic: if the pendulum flips over, we are back to where we started and this is still periodic motion.
2. The motion comes to a stop. This is not the case here because $\ddot{\theta}$ and $\dot{\theta}$ do not have the same zeroes, except in the cases where it starts at $\theta=0$ with no velocity or has exactly the right velocity to come to a stop at $\theta=\pi$.
3. The motion is periodic.

So, except for the special cases where it comes to a stop, this describes periodic motion.

On the other hand we have: $$\ddot{x}+\gamma\dot{x}+\omega^2x=0$$

First off, it is clear that our solution will not have velocity be just position dependent, because the acceleration is velocity dependent. To investigate this, it is helpful to look at a similar quantity to the conserved quantity from the last part:

$$\ddot{x}+\gamma\dot{x}+\omega^2x=0$$ $$\ddot{x}\dot{x}+\omega^2x\dot{x}=-\gamma\dot{x}^2$$

$$\frac{d}{dt}\left(\frac{\dot{x}^2}{2}+\frac{\omega^2x^2}{2}\right)=-\gamma\dot{x}^2$$

Notice that, if $\gamma>0$, the term on the right is always negative, and so the "energy" of the system is always decreasing!

Now if $\gamma$ is small, we can imagine that we get mostly periodic motion just as before for the same reasons (note that the "potential energy" part is unbounded, so it can't go to infinity), but the energy drains out of the system over time (if $\gamma>0$, otherwise energy is being pumped into the system), so the amplitude of each successive oscillation is smaller. If $\gamma$ is large, we don't even have periodic motion initially- the energy is quickly dissipated.

I would do it by noting that the first system is conservative while the second is not.

Thus, for the pendulum, $$ E=T+V=\frac{1}{2}m\ell^2\dot{\theta}^2+mg\ell(1-\cos\theta)\, . $$ Recognizing the $E$ is constant and can thus be evaluated at the turning points of the (bounded) motion where $(\dot\theta,\theta)=(0,\pm \theta_0)$, we have $E=mg\ell(1-\cos\theta_0)$. Thus, reorganizing and keeping in mind that $\dot\theta=d\theta/dt$, we reach \begin{align} dt&=\pm \frac{d\theta} {\sqrt{\frac{2}{m\ell^2}mg\ell(\cos\theta-\cos\theta_0)}}\, ,\\ T&=4\sqrt{\frac{\ell}{2g}}\int_0^{\theta_0} \frac{d\theta}{ \sqrt{\cos\theta-\cos\theta_0}}= \sqrt{\frac{\ell}{g}}\,f(\theta_0)\, . \end{align} Of course for the second system you cannot do any of this as energy is not conserved.