If you want to check for changes in the Site Audit tool, there are a few ways to do it.
The issues report:
Shows you changes in highlighted columns. The changes are compared with the previous crawl only.
Projects overview report:
From the overview report, you can see changes in your Health Score and internal URLs with errors.
While the bigger graph shows you changes for previous weeks/months (depending on your subscription), the other ones are showing changes since the previous crawl.
Page/Link explorer reports:
If you click on the button "show changes", it will compare the changes with the previous crawl. This feature is only available in page explorer and Link explorer for now.
By clicking on "manage columns" you can adjust what kind of info you want to have displayed in the report.
Q: Can I re-crawl URLs with a specific error to validate fixes?
A: Unfortunately, we don't have the option to validate fixes without running the crawl again.
Q: Do you show fixed errors instantly?
A: After you fix the errors or warnings, most of them will appear as fixed in the next crawl. There are cases when it does not appear instantly, i.e. if you have a cache time set to be refreshed every 24 hours and you make the change on your website, it might not be reflected until the next day.