Manuscript evaluation for becoming an editor
Different editors treat this differently. Some of the things I looked at:


Does it look like a serious research paper? If not, desk reject.
Does the paper pass plagiarism detection? If not, desk reject.
Is it plausible? The methods used should conceivably lead to the results. If it's not plausible, unless the authors provide a convincing explanation, desk reject.
Is the paper review-ready? A paper that has lots of typographical errors, has figures without axis labels, is written in incomprehensible English to the point where I cannot in good faith ask reviewers to wrack their heads deciphering the paper, etc - reject and resubmit.
Is it within the journal's scope? If not, desk reject.
Does the paper have anything interesting to say? Chances are I won't be able to judge the significance of the paper myself, but the authors ought to have written something in the introduction about why their research matters. Something like "this material is important because of [reasons]. Nobody has ever done [calculations] for this crucial property. We do it in this paper and suggest [methods] to make an even better material." would be helpful. If the authors don't give a reason for why their research matters, a desk rejection is more likely.
What tests did the authors conduct? How do the authors know their results really are reliable? If no error analysis is performed, desk reject. The same goes for any standard tests, e.g. before one can claim that a new drug is effective, I'd want to know if there was a control group, if the results are statistically significant, etc. If the authors do not discuss these standard things - desk reject.


Beyond this it depends on what the reviewers say. Reviewer reports can be very varied so it's hard to make statements about them, but the idea is the same: I read what they say and come to my own conclusions. It is possible that I don't understand what the reviewers or the authors are saying, in which case things can really seem like a coin flip. Peer review isn't a scientific process. As a personal guideline, when in doubt, I accept the manuscript. Another person might lean towards rejecting.