There is kind of a difference between "wasting away" physically vs viscerally, I guess.
In the visceral sense -- I'd say in order to be diagnosed with that type of malnutrition, you need lab values. This would be protein values in the hole, including albumin, prealbumin, total protein. Some people also experience edema...lower legs or tummy area.
You could be physically malnourished, which would be detected physically from loss of muscle tone. The physically malnourishment would be caused from long-term calorie deficit. Basically, you'd be skinny, and lacking muscle. You can kinda see these places on the body - hands or temple area are pretty common places to look. Having an indentation in the temple area is a pretty good sign...because there is typically a small muscle that should sorta "fill in" that area, but if your body is breaking down muscle, that muscle gets broken down and the skin starts to indent.
So, you could have one, the other, or both (really bad!).
And then, each of those would be different than having low body fat. When I think of malnutrition, I think of protein and muscle.
So, if you think your lab values are scary, the first place to start would be to check those. That'll tell you whether you're getting enough protein and stuff.
Then, if you think you're getting too thin, it might be time to consider some digestive enzymes to get you by.
I'm thinking based on what you say, you might just be getting a little too thin...maybe losing too much fat or muscle mass for your liking. How are you feeling otherwise?