How can I spot design mistakes in my own resume?

Look at it in several different ways, because you get used to your own document and stop noticing problems. A short routine catches most issues.

Checks:

• Squint test: blur your eyes. Can you still see the sections and job entries clearly?
• Alignment check: run your eye down the left edge and the date column. Is everything straight?
• Consistency check: read only the dates, then only the job titles, then only the headings.
• Phone check: open the PDF on your phone. Is it readable when zoomed?
• Print check: print it in black and white.
• Copy check: paste all the text into a plain text editor. Is the order sensible?
• Fresh eyes: ask a friend to find your current role and top skill in a few seconds.

Fix what you find, then repeat the checks on the final PDF.