One way to test it out (not an option for everyone) is to fill it with smoke. If it clears within a few hours, you have good ventilation. If it doesn't clear, and air tends to stagnate and just sit there inside, that's another condition that promotes respiratory infection, especially when moisture is present.

If the enclosure dries out fairly quickly, then your ventilation is probably fine because even with plenty of heat, it wouldn't dry out quickly without good air flow and you would notice condensation in spots on the walls.