Unfortunately, measuring it is somewhat technical and requires equipment few people have.
If you place a paper towel on the carpet and stand on it for 30-60 seconds, it should not feel damp. That's dry carpet.
In some climates, getting carpets dry quickly enough is fairly easy. In others, it's very hard.
Without a truck-mount, airmovers, possibly dehumidifiers and expertise, it can be dang near impossible.
I've seen a couple of carpet cleaning folks in my time, with and without the truck. I think "expertise" may not be the appropriate term here. I'll grant that truck-mount vacuums are generally more powerful than plug-in models, and air movement to help pull the moisture out of the carpet presents a useful tool, but the guys I've seen cleaning carpet range from pretty intelligent to those that are lucky to be able to plug the vacuum hose in properly. Yet, they still manage to clean a carpet and have it dry in a timely manner. They always recommend 1 day before removing the little furniture coasters regardless of their skill. I imagine that represents covering for the ones that soak a carpet and fail to pull the water back out.