Yeah, it's subjective. But the brain (most people's brains) are wired to express emotions with the same facial expressions, though this is modulated somewhat by culture. Brains also generally have "mirroring" circuitry--people can tell, in part, how someone else is feeling because their brains automatically imagine what they would be feeling to have that facial expression (for instance).

In some people, this mirroring is weak; I'm guessing that is true for you. But you could try looking at yourself in the mirror while thinking about different things, and then compare your facial expression to the ones other people say are associated with different feelings. There may even be some book or other resource used to teach autistic kids these things that you might find helpful.

Of course, your innate facial and body language expression may not be standard, but it's nonetheless a place to start. And facial expression, posture, and the like, are at least objective, even if they are interpreted subjectively.

Like you noticing you slowed down when you thought about making a phone call--the depression was subjective (i.e. internally experienced--the chemical state of your brain is, of course, an objective reality, even though you have no way to put a sample in a test tube to determine what it was), but the slower walking was objective, and didn't take any special training or equipment to notice.

(Most people don't realize it, or perhaps don't like to think of it in cold scientific terms, but emotions are biochemical in nature.)