Definitely by comparing to older work! Just last night I was comparing the earliest pages of Grassblades to the ones I'm currently drawing, and feeling proud that I'd levelled up.
When I started Grassblades, I agonised over drawing architecture and more than two characters in a panel - these days, I draw crowded castle courtyards and complex architecture and I don't even have to think about it first. It takes a long time, and I still make mistakes, but I do it without having to convince myself first.
As for writing... I think it helps me that I tend to work on Grassblades a whole chapter at a time. It means I'm able to go through a bunch of different drafts of the dialogue, and really consider the dramaturgy of each chapter as I plan it out. I'm able to pick up different plot-threads and weave them into what's happening in each chapter, hint at stuff, give information to readers piece by piece, etc.
Looking at older work for comparison is good for writing as well; whenever I feel like I suck at it, I pull out the terrible older comics I've made and comfort myself with the thought that at least my dialogue is no longer that awkward, at least my characters are people now instead of cardboard stereotypes, and at least I know where I'm going this time, and not letting the main plot just disappear on me.
As a general rule, I've noticed that moments when I'm struggling with drawing/writing something, and everything comes out wrong, is usually the moment right before I level up, so to speak.
It's like I've suddenly gained the the eye for spotting my flaws, which is the first step to getting rid of them.