Then, right in the Introduction, you can often recognize whether the authors considered the full context of their topic.
After that, I check whether all the experiments and data make sense, paying particular attention to whether the authors carefully designed and performed the experiments and whether they analyzed and interpreted the results in a comprehensible way.
Having said that, I tend to define my expertise fairly broadly for reviewing purposes. I am more willing to review for journals that I read or publish in.
Before I became an editor, I used to be fairly eclectic in the journals I reviewed for, but now I tend to be more discerning, since my editing duties take up much of my reviewing time.
Unless it’s for a journal I know well, the first thing I do is check what format the journal prefers the review to be in.
Some journals have structured review criteria; others just ask for general and specific comments. I almost never print out papers for review; I prefer to work with the electronic version.
I see it as a tit-for-tat duty: Since I am an active researcher and I submit papers, hoping for really helpful, constructive comments, it just makes sense that I do the same for others.
So accepting an invitation for me is the default, unless a paper is really far from my expertise or my workload doesn’t allow it.
I read the digital version with an open word processing file, keeping a list of “major items” and “minor items” and making notes as I go.
There are a few aspects that I make sure to address, though I cover a lot more ground as well.