At least once a week I will find a dryer vent that is torn or disconnected somewhere in the crawlspace. A dryer venting into the crawlspace will introduce a significant amount of moisture into this area & this is never a good thing. The quickest & easiest way to check to check to see if your dryer is venting to the outside is to turn your dryer on & go locate the vent termination on the outside of the house & verify that there is warm air coming out of it. If there is not it is time to crawl under the house & locate the problem. I also recommend getting rid of any flexible vinyl duct & replacing it with metal duct that has no fasteners penetrating the duct that can catch lint & the joints running in the direction of airflow.