As an example, I'm going to use a thread that was recently edited: https://www.mathworks.com/matlabcentral/answers/665533-how-can-i-plot-this-type-of-figures
Remove everything after the thread ID and paste that in a Google search:
That gets us these results. You can get to the cache by clicking the arrowhead next to the URL.
First confirm we are looking at the correct thread. We searched for 665533, so that should be in the URL in the cache as well.
This checks out: we are looking at the correct thread. Unfortunately, this cache is from after the tampering.
Returning to Google, we see the UK mirror shows up as well. Sometimes you need to click the ‘repeat the search with the omitted results included’ button at the bottom of the page to see all mirrors. In this case clicing that button I can find another result from the UK mirror, but also the Swiss, Italian, German, and Korean mirrors. Sometimes these are updated more slowly, giving you multiple chances to find the latest version before the tampering. Sometimes the arrowhead is missing, which means there is no cache available for that version of the page.
In this case we can use the UK mirror that was the second result.
The thread ID matches, so this is indeed the correct thread. It is also from before the tampering.
Now let’s create a permanent archive, because the Google cache will be updated at some point to reflect the live (and thus tampered) page.
I prefer to use the Wayback Machine, which you can access on web.archive.org. You can prepend http://web.archive.org/save to the URL, but you can also navigate to web.archive.org/save to get the page below.
Paste the Google cache link in the box (optionally uncheck ‘Save error pages’) and click the ‘SAVE PAGE’ button.
Now you have a permanent copy of the cache, which you can use to copy the original question content from.
Don't forget to put that link in your post, as that is the only easy way to get to the saved cache.