I think I found my answer.
cbi(DDRA, PA1);//set A1 as input pin
//if sensor output is 0, do something
if(bit_is_clear(PINA, 1))
take_over_world();
Where PA1 is whichever I/O pin I decide to use? Do I need to declare the digital pin in the header file, or are all unused pins automatically set to accept digital input?