- Four vents exist on the retina Macbook Pro 15".
- Two vents exist on the non-retina Macbook Pro 15", 17" models.
- One vent only on the Macbook Air 11" & 13" and non-retina Macbook Pro 13".
The outflow vents are located right 'between' the display and keyboard on the vertical rear face of the unibody frame. Shine a light on the display hinge from above and below and you can see the little holes best when looking down along the display from a very steep angle.
While the non-retina MBP take all of the inflowing air from the remaining space next to the outflow vents of the display hinge - the retina Macbook Pro has an additional two vents for air intake on the side where external ports are located.
Apple has done an amazing job at the dB noise they make when idle (~2000rpm).
When the fans kick in because of heavy CPU/GPU usage you'll definitely hear them. They will go up to ~6000rpm. Tracking fan RPM with software will be your best method to know if the case you have is insulating the frame. Increased idle fan speed will compensate for a case that insulates the frame and traps heat from below. The unibody frame in in close thermal contact with the CPU/GPU so a good proportion of the idle heat actually escapes through the keyboard and topcase.
You might also be interested in Apple's advice for Mac care.
[picture from ifixit]