Latexdiff

latexdiff old_version.tex new_version.tex > diff.tex
makes diff.tex contain the marked up differences (red striked out for deletions, blue underwiggled for additions).

• It is also available directly on Overleaf. The created difference LaTeX file can be accessed (if necessary) via "Logs and output files" → "Other logs and files" (at the bottom).
• ctan-README
• things it may choke on:
  • missing \\ before \hline