Things to look for for factory air:

Big hole in the centre of the firewall (for vacuum supply lead and A/C wiring), approx 32mm.
Overlapping 16mm holes or slot down below the heater housing for heater core spouts.
Where the cowl fresh air vents screw into the plenum from below on both sides, non-air cars have them held in with 2 x screws. A/C cars use 3 x screw holes (driver's side one is a blanking plate on A/C cars).