How do i know prostate contact is established

In my case, at least, the prostate doesn't have any distinct feeling of contact — not like skin for instance.
You'll know it's being massaged when pleasurable feelings start happening. Aside from that, yeah, it could feel like the Aneros is touching nothing.
The obvious way to see is using your finger when lubing. Half way down the second joint of your finger, you should be able to feel a small bump. That'll be the prostate.

Being a newbie, when I first started using I could not tell if it was touching my prostate. Now, after around 20 sessions, I know it's touching, I can feel it and what section it is touching. I think I needed to awaken my prostate. It felt gradual. At first I felt something but I wasn't sure what it is was. Over time I established more response and feeling. So to answer your question, I think you need to awaken your prostate, and that should happen gradually over time.

After years of use I still can't tell if it's touching my prostate.

At first, I couldn't tell either, but now after several years at it... there is no question, it's touching. Having put my finger in there, I can say with certainty that your prostate is very easily accessible, and close to the entrance. It's pretty much impossible for any prostate toy to NOT be touching the prostate.

I'm not convinced!
I've put dildos and my finger and aneros up there and dont feel it.
Besides, there's a wall, sometimes thick, between the anal canal and the prostate, so you dont touch the prostate directly and perhaps the wall is too thick so that you cant feel anything.

I was getting my prostate exam by the GP quite few years ago, I don't get them any more. But anyhow as I was leaning over that stirrup bench, and the doc put his finger in my anus, boy did I feel it when he hit the prostate. I fell in love with his finger right there, almost shot my wad. I've been looking for that feeling ever since. I do get a somewhat sex/good feeling with some toys, mostly the MGX, got a eupho recently, I'm thinking the bigger ones like the maximus don't move around enough. And yes if it's only finger depth inside, it's not far, maybe 2-3" in at most. I'm also thinking it's like tickling yourself, it's hard to do, may be easier for someone else to find it, but my wife won't go there. I've had no luck with my finger, I can't get in in enough from in front and from the side/rear around the cheek, I can't reach far enough, plus it's sideways. You'd think the Peredise would work, but they don't do much for me, yet.