“How do you know which of your memories are genuine and which have been altered over time or even made up?”
Another superb question!
My opinion comes down to realizing that not all memories are created equally.
Every one of us could experience the exact same event and recall it differently.
I wrote a post recently showing that the human mind can only focus on one thing at a time with efficiency. What we focus on is entirely personal.
If something happened directly to you, I would think a person gathers more sensory information but that may hold an exception. When we are in a heightened emotional state, such as pain, a human being would likely focus on the pain at the exclusion of collecting information. That’s why a witness to a mugging, IMO, would have more information to share than the person who was mugged. But, eyewitnesses are terribly unreliable too. That ‘one thing at a time’ aspect means excluding many peripheral details is any person’s limitation. So, many witnesses are preferable at a crime scene, and just like doing research, the intersecting and repeated reports are likely more reliable. BUT, if emotions are running extremely high, all bets are off because a whole mob can become convinced that they saw something like the “Hands up, don’t shoot.” narrative at the Michael Brown shooting.
Personal memories are a little different. I think we all aggrandize those. Our feelings are applied to them and we prioritize their importance ‘letting go’ of those not as emotionally friendly to us.
Then we ask, are some people’s memories a little more reliable than others?
I’d say yes.
There’s eidetic memory. It’s what we call ‘photographic memory’.
Even in that category, some are better than others.
I’ve heard that some people can recall almost every day in great detail! That’s incredible!
For me, I do have a collection of detailed pictures that I can call on. (And until recently, I believed everyone ‘sees’ pictures.) They’re like photos (sometimes short videos) taken in moments that made an impression on me. Having those, makes me a guarded more reliable source if you pick a topic that I chose to memorialize. And, what a person chooses to remember has limitations according to the person’s preferences so do research yourself on topics of importance because no one is a single reliable source!
The only reason it can be more reliable to have a visual memory is that a mental ‘picture’ offers extra clues like those multiple witnesses did to a crime. If I ‘see’ my brother riding his bike in a memory, for example, I know where he was and what the bike looked like. Those are great clues if I wanted to know how old he was in the memory. I can investigate when he had that particular bike, and when he was at that location. Also, the picture probably gives away the time of year and possibly the make of a car in the background or family pet. and those would also narrow down the year.
Memories are delightful companions. Human beings ‘dress them up’ a bit for extra fun and because we all wish for positive experiences the ones we choose are usually beautiful. They’re probably our best defense against horrible incidences which seem to tattoo themselves in our psyche liking to show off at 2:00 am. 😉
Fandango’s Provocative Question #17 Revisited – This, That, and The Other (fivedotoh.com)
Sillyfrog's Blog 
I think you’re right about the unreliability of memories and how eye witnesses can see the exact same thing but remember it differently.