How Can You Extract Numerical Values from Output Box Data in Mathematica?

[Orion1]

$$\text{In}[1]:=\text{Solve}\left[\partial _{\theta } \text{Sin}[\theta ]==0,\theta \right]$$

$$\text{Out}[1]:=\left\{\left\{\theta \to -\frac{\pi }{2}\right\},\left\{\theta \to \frac{\pi }{2}\right\}\right\}$$

Is there a Mathematica source code command that can read a specific numerical value from the output box data?
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[Hurkyl]

The easiest way to extract the target from a replacement rule is to just use the replacement rule.