Conversely, is it possible to physically or remotely access one of these machines and determine if they've been hacked (or pre-hacked?) and how long would that

Conversely, is it possible to physically or remotely access one of these machines and determine if they've been hacked (or pre-hacked?) and how long would that take? Also, does the hack simply modify the results for that machine, it does it grant access to manipulating the entire county vote?