There isn't really any such thing as 'false memory syndrome'. Sometimes people have memories that turn out to be inaccurate, but it's not a syndrome. That was invented by an organisation funded by people whose children accused them of CSA. I think we've talked about this before, I'm not sure but thought I'd mention it again.

How can real memories of sexual abuse be distinguished from manifestations of false memory syndrome (FMS)?

My personal opinion is that this comes down to you. Your own perceptions and beliefs. If you clear away everything other people have told you to think, the 'official' story of your life, and just pay attention to your own experience and perception, you'll find it easier to work this out.

It is my personal opinion that this question does not come out of nowhere. Most people who ask it seem to eventually discover an answer, one way or another. How should you handle it? It's taken me a while to see this, as I went through a period of torment where I wanted to know what it was that I'd blocked out, but actually the exact content of your memories isn't the most pressing thing - it's more about how you feel about them and how you are affected now.

My T and I have discussed how I'm pretty sure of what happened to me but my memories are missing, fragmented or hazy - my T's objective opinion is that something did happen. But the place we've reached is to realise that it may as well have happened as the effects are the same. Obviously it matters if it did or not because it's a parent in my case, and nobody could happily sit with the torment of not knowing either way, so it does matter whether it really happened (and I'm still going round in circles where I try to convince myself I've made it up even after having some flashbacks).

In your case, your memories, whatever they are, wherever they've come from, are troubling you and they contain information. And I think the nature of so-called FMS - or the situations they ascribe to FMS - is that therapists have supposedly implanted 'memories'. It sounds like that is not what has happened here.