How do I know how well I am progressing in my PhD?

In my observation over some decades (in mathematics), I think it is nearly impossible for a PhD student to gauge their own progress. Sure, it is possible to know whether one feels discouraged or encouraged, etc., but these are mostly artifacts of one's own temperament and of the general environment, not indicators of one's progress.

If you have an engaged, perceptive, competent advisor, they will by-far be the best judge of whether you're on-track or off.

Even then, the real goal is not to measure up to some scalar-valued "good/bad", but to _become_what_you_can_.

I can tell you how I measured my progress in an area (Theoretical CS) where the few papers arrive on the final year of the PhD. The first year of my PhD I studied a lot. I mean a lot.

And I realized I was making progress when I could follow arguments and expositions that were too alien for me in the beginning. When I was reading a description of a theorem (not the proof) or an algorithm and thinking "Ah, this is not surprising because of such-and-such reason". When initially heavy subjects became part of my everyday routine.

When I arrived to that point, I was confident I was on the right track even without any publication.

Ask your advisor.

I mean, you can even forward the question to him in an e-mail... That's what advisors are for. Ask him if is there anything else you could be doing, what can you do to improve your performance. Literally nobody else would be as informed as him at this point.