There's no sensor for the injectors. The computer can tell if current is flowing when it's trying to power an injector. An open circuit can be caused by a broken wire, bad plug, or the injector itself having crapped out internally. Unfortunately the ECU can't tell you which injector isn't taking power, so you'll have to test each one, though with a little digging in the manual you could track down which wire goes to which injector at one of the plugs (pretty sure that plug is on the passenger's side valve cover) and test from there. If you haven't downloaded it yet, get the service manual from nicoclub. The EF&EC section is your friend for most engine issues.