But you assess digestion by all sorts of factors like Transit time (eat corn and see how many hrs it takes to come out the other end) should be 8 hrs give or take a few hrs.
The quality of appetite, if its weak, irregular it means low digestive fire, and this person will not be producing adequate enzymes.
Tongue diagnosis is not as complex as pulse diagnosis, you can pick up a book on both.
If the person has strange cravings that is a sign of illness or deficiency. Ive noticed 2 types of people. Those who crave foods they need to fill a deficit and those who crave bad foods. (this is similar to sex drive, a high sex drive can be a sign of health or disease (good horemone levels or disease irritating toxins)
Here is something I found a while ago, kinda funny but its a way to assess one aspect of digestion
a good digestion should be between level 1-2 but easy to pass