I'm repairing the Magna, the air bags never went off.

Does anyone know what sets the front air bags off, the side ones are listed in the manual.

What precautions should I take before connecting the battery?

Precautions? Ensure nobody is seated within range of an airbag lol

Best bet might be to find the workshop manual thread, and download the Genuine Mitsubishi workshop manuals for your model - they are PDF files hosted elsewhere by one/some of the members. these are excellent resources, as they are the manuals a dealer service dept would use and be trained from.

From memory, 2nd gens had a sensor near front of car under radiator or similar.... pretty sure the 3rd gens have only the ABS ECU in the cabin, with sensors in it -if SRS has been activated, the ABS ECU needs to be replaced. If bags didn't fire, then you are probably still able to use that ECU - if SRS light behaves normally that'll be the real clue.

I've only ever read the TJ Manual for when I put the ralliart steering wheel on TJ. I'm about to fit steering wheel audio controls to my KJ, so will have to re-visit the manual about bag/clock-spring etc.

If the car is reassembled, then no special precautions about reconnecting battery AFAIK.

I found the workshop manuals, but in the diagram they only show the sensors for the sides, it doesn't mention what sets the front sensors off.

I'm pre-side-airbags - only in a TJ and KJ.
From my reading of manuals for TJ/KJ there are no seperate sensors for the driver and passenger airbags, the crash detectors are inside the SRS ECU box inside the cabin. If you are in a more recent vehicle, it may be the same.... ECU unit for SRS contains the frontal crash sensors.