But how do i know if the members in their forums are not fake accounts of their staff or something like that ?
Maybe they pay people or smth else .-.
I guess you don't know for sure, just the same as you shouldn't really trust anything you read online. But I don't see why that matters. What you're looking for is info. You can get some info there (and elsewhere) and make your decisions. Even if someone misleads you, you're only risking what you hold on the platform at any given time (see my post above). Buy 0.1 BTC with your credit card and withdraw it to your wallet. Or transfer 0.1 BTC, sell it and bank your profit. At the end of your first transaction, you will have a balance of 0 on xCoins.
That way, you'll see how it works and you only risk 0.1 BTC. Then, you can decide whether you should continue. This is exactly how I started on the platform, and I've done a few $K in volume now with no problems so far (though like I say, I've been lucky).