How should I test my home's humidity?
A Hygrometer is used to test the level of Humidity. I'll bet that your levels are very high. High humidity can lead to health issues with mold groth, not to mention it can destroy your home. Things you can do:


Vent the dryer to outdoors, a big source of the problem!
Add a bathroom vent to the outdoors, not in an attic or other room! Run the fan for 30 minutes during and after the shower (timers are available).
Vent the range hood if possibly (least of all, but still a good idea).


If your levels still test high, you can add a dehumidifier or a more sophisticated HVAC solution. I keep my home @ no more than 40% relative humidity. Check your local for recommended settings.

Exhausting the air requires that the air be replaced with fresh air. This can be as simple as someone entering the house from outside to a system that replaces the air from an intake ported from outside. I'm not suggesting you need to go that far, unless your budget allows and your problem is that big. Often it is enough intake just from normal leakage from windows, doors etc., However with newer housing becoming increasingly tighter, some situations require the expensive solutions. The big negative of not having enough intake is that appliances like gas stoves, fireplaces, gas water heaters, gas furnace, etc, will starve for fresh air intake, from the negative pressure created. This will make the combustion incomplete causing deadly CO to escape into the home! So it is so very important that all homes have at least one CO detector!