There are a a couple signals you should look into house ventilation, starting with:
1. Ever-present odors
If you have issues getting strong aromas out of your house, you might have a ventilation issue.
2. Musty air
The air in your space should seem fresh, but musty areas mean you might have inadequate ventilation.
3. Lots of dust
Dust can develop more rapidly in homes with inadequate air movement.