Sensing 12v inputs in 3.3v system
If everything is at a common ground, then a simple resistor divider might be good enough.  However, there can be nasty spikes on the 12 V power.  If these spikes can get to the 12 V signals you are trying to detect, then you should include some additional clamping circuitry.

All around, a opto-isolator might be a good idea, especially if this is a one-off where you're not worried about volume production costs.  You probably don't need the common mode isolation the opto provides, but it is a simple way to just not have to worry about ground bounce and the nasty power spikes.  I'd go with the opto solution to play it safe.  It's really not that much more complicated or expensive.