Which side of my gas furnace is the vent?

The one with the plastic (honeycomb) disc is for intake air. The other is the vent. To verify - Turn the gas shut off valve to the closed position and turn the furnace on. The one blowing the air out is the vent and the one pulling air in is the intake. No this won't hurt the furnace.

To the best of my knowledge, gas company code requires a double-wall pipe designed for gas furnaces. They will get hot to the touch, even to the outside layer. In my new house, I added a metal sleeve around the double-wall pipe just as an extra safety precaution so I could run insulation up against it. My son's house has a new high-efficiency gas furnace that has a 2" plastic pipe for the spent fumes. It gets kinda warm to the touch but never hot.

Usually the perforated disk is the intake.

The exhaust is the one connected to the venter motor which pushes exhaust out, the intake goes to the burner area.

The wide open side has got to be the vent. Heat from the vent would harm plastic. The plastic side must be for air intake

Dig just a little deeper into the unit,.... with just the first layer of sheet metal cover(s) off you should be able to see which one is ducted to the burner, and which one is ducted from the top of the heat exchanger into the flue.

Likely the wide open side, PVC is not normally used for venting exhaust, it gets hot. Can you refer to the installation instructions?

I would just leave it to the pro's gas is dangerous, if you have a good insurance cover for your boiler calling the gas man out should be covered.

the screened pipe