How do you know that your memories are true?

As it turns out, our memories are not true in the sense that we believe they are. In what we would think of as a true memory, we would have something more like a video in our minds, one that recorded everything we were experiencing.  In fact, every time we pull out a memory and dust it off, it changes a little, getting a kind of overlay of the new person we have become since we last retrieved it.  This is how we remember people whom we do not see too often. Did George go bald? It is likely that your memory has picked up on that and stored it where his long hair used to be.  And our memories are completely selective, such that two people can be at the same event and remember entirely different people and actions.  The more we learn about how memories are formed and retrieved, the more we come to realize how unreliable they really are, one of the reasons that eyewitness testimony has resulted in many erroneous convictions. 
Having said that, though, I will say there are ways you can check on various facets of what you remember. Another person might confirm by having the same or similar memory.  A newspaper account might include something you think you remember, and you can check that way.  Some people keep a journal where they write down various "facts" they can check on later.  What you will often find is that people do have slightly different versions for themselves. If I didn't know better, I might swear that my sons were raised in completely different households, based upon what they remember.  But it is in the overlap of memories like these, what people agree happened or was said, that you can sometimes confirm what you believe you remember.  At the very least, they agree on what their parent's names are and where we went for family vacations when they were young. 
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3341652/

https://www.scientificamerican.com/article/scientists-plant-false-memories-in-mice-and-mice-buy-it/?redirect=1&error=cookies_not_supported&code=bfa13ca7-d14c-41fb-8be3-99203e51a0df