How can I prove that the NPN BJT is in the active region?
After measuring all the voltages, I got: \$ V_{BE}\approx 0.6V, V_{CB}&gt;0 \$. Are those measurements sufficient to say that the transistor is operating in the active region?


No, it's not sufficient. You could measure the same two conditions if the BJT is in saturation.

If \$V_{CE} &gt; ~0.2\ {\rm V}\$, then it's very likely the part is in forward active mode. If \$V_{CB} &gt; 0\$, as you said in comments, then it's safe to say the device is in active mode.

But the definition of saturation is somewhat fuzzy, and might be defined differently by different manufacturers or for BJTs designed for different applications. Also, it's usually defined in terms of current, not voltage. A typical definition of saturation is when \$\beta &lt; 10\$ (or 20, or some other value). So to prove the BJT is in forward-active, you'd need to work out the base and collector currents, and show their ratio is above the threshold you've chosen to define saturation.

Theoretically, there's nothing really to prove. You just have to make a clear definition of the boundary between active and saturated, and then see whether your operating point matches that definition or not.

Note: I made a mistake and gave the condition \$V_{CB} &gt; 0.2\$ instead of \$V_{CE}&gt;0.2\$ when I first wrote this, which might have led to much of the discussion in comments. Now corrected.