The vent is connected to the drain, you won't see anything on the toilet itself. If there is water in the bowl after flushing, the smell is not coming from there. If air is going in the vent when you flush, it's vented (although may be partially blocked).
When drain lines aren't properly vented, low pressure inside the pipe can suck water out of the traps, letting sewer gas in. Air pressure imbalances inside the drain pipe can restrict water flow and cause sluggish draining, often mimicking a partial clog inside the pipe.
If you have downward-sloped pipes, your toilet will most likely work without a vent. Gravity does a lot of the heavy lifting anyway.
Take 2 pieces of toilet paper. Turn on the fan and place the toilet paper against the fan cover. The fan should suck up the toilet paper and stay attached to the fan. If not, your bathroom exhaust fan isn't working efficiently.
The plumbing vent/s usually go though the roof, and can become clogged. Usually there is one main vent close for toilets/sinks/showers, but if there is more plumbing at the other end of the house, might be a second vent. Try to find a clean out(a tee with a cap) and can probably run a snake up the vent.
The most common configuration is to feed 2" PVC down from the ceiling within the wall behind the toilet. The vent pipe connects into the toilet drain pipe. The sink drain pipe and the tub/shower drain pipe are vented with 1.5" pipe that branches off from the 2" PVC mainline.
Excess Air in the Water Tank
Just open the filler valve at the back of your toilet and wait for about five minutes while it drains out some of its own water, then try refilling it again. This should get rid of any excess gas rattling around in there.
Check the water level:
This cause by far the most common reason for slow flushing toilets. If the water in the tank is not up to the recommended level, you may have poor flushing power.
Poorly-vented drain lines will not be able to effectively move wastewater and solid waste out of your building. This could lead to problems such as overflowing drains, backed-up toilets, and similar plumbing issues.
Behind or within the Wall: In most cases, the vent pipe for a toilet should run vertically from the toilet's waste pipe (known as the closet bend) and extend upward to connect with the main vent stack or a branch vent line that ultimately exits through the roof of your home.
The cause of your toilet gurgling is a blocked line somewhere. A clog is creating negative air pressure, Instead of air flowing through the lines,the air pushes back and causes the gurgling sound. Sometimes, you'll also see the toilet water bubble.
Use a garden hose to gently flush water down the vent stack and observe if the water drains properly afterward.
The easiest way to check if the fan is ventilating properly is to take a Kleenex or single square of toilet paper and place it along the fan while it's running. If the fan holds up the paper and keeps it attached, then it's working properly.
Signs that your bathroom needs more ventilation are:
Stuffiness when you enter the room. Mildew on walls or tiling. Stains forming on walls or ceiling. Smells that linger for a long time.
Use a plumber's snake of electricians fish tape to unclog the stoppage. If you can't entirely remove the clog with your “snake,” use a garden hose to flush out remaining debris. Have your assistant flush the toilet again while you hold your hand over the vent to feel for suction.
If you decide to hire a licensed plumber near you to handle the fix, you can expect to pay between $150 and $200 for this service, although it could be more depending on the severity of the problem.
Dry and Empty Toilet Tanks
The exact opposite can happen if the plumbing air vent gets clogged. That's right — you'll have no water in the tank to flush away what you need to.
If you have an internal bathroom, you can set up a room dehumidifier or install a ventilator. Or, if the bathroom is favourable located in your home, you can open the bathroom door and open the windows in the adjoining rooms to create a flow of air.