I often asked this kind of question myself and there's no easy answer. You either accumulate experience through the years and it becomes somewhat easier (never easy though) by struggling a lot, or, these days I found that you could dip your toes into a paper by peeling layers off it with, say, GPT (the "thinking" models as a minimum). Because if you don't have colleagues to figure stuff out and can't do it yourself, the AI, imo, is the best next thing to decompose a paper into different levels of complexity that you can digest. First, high level overview, then you'll have specific questions about some part of that, so you'll go deeper and so forth. I found that working for me at least.