Get User:Dr pda/prosesize.js, a tool for counting article prose size. (That page has instructions for how to enable it in your browser.)
If you would prefer not to use this tool, you can calculate article size by hand using a word processor, by simply copying and pasting the prose portion of the article. The prose portion excludes wiki markup, templates, lists, tables, and references (thus, to do a character count by hand, you should remove those parts of the article from your count).
Open the article you would like to check and count the prose size using the prose size tool installed in the previous step. Save this number for later.
Open the page history of the article you would like to check.
Find the last edit before major expansion began. (This is usually, but not always, the last edit by some other author before the DYK author started expanding the article.)
If that edit is within 7 days of when the article was nominated, proceed as normal.
If that edit is more than 7 days before the article was nominated, this means you must instead calculate expansion from a more recent date. Instead of using this version, find the earliest version that is within 7 days of the date the article was nominated, and then proceed to the next step.
Open that version and count the prose size using the prose size tool installed in step 1.
Compare the number from step 2 to the number from step 5. If the first number (the current size) is five times greater than the second number (the article size before expansion, or the article size 7 days before nomination), the article meets the fivefold expansion requirement.