Connecting smart switch, identifying neutal wire
This is a switch loop.  The unpainted whitee and black are always-hot and switched-hot.  

You don't have neutral here.  Further, the extra hot wire can't be on this switch.  

Notice how the hot and switched-hot on this switch do not have any paint on them.  They were added later, and are a switch loop to a different circuit.  They were installed before NEC 2011 took effect, because that requires neutral on a switch loop. That would be the neutral you need and do not have. 

The paint-speckled hot, however, is a dangerous interloper.  It is stealing hot from this circuit, but returning neutral on another circuit, causing that neutral to be overloaded*. 

Further, that invasion is totally needless.  There are plenty of wires back there, so some must surely be hot for its own proper circuit.  This needs to be fixed, by a pro if necessary.

Regardless, given the availability of power here, the switch loop might be "flipped", so the power at the lamp is not used by the lamp anymore, and the power comes out of this box.   If that were done, the two extant wires would become switched-hot and neutral, and you would be able to install a smart switch here.