You do not, so you do not get to trust anything or anyone. That's the exact point of being your own bank, as you are with Bitcoin; you do not trust, rather you verify.

Do not even trust that your friends are "making" money out of it, a quick search on how to mine Bitcoin would have revealed how a Bitcoin mining app is 100% a scam and would have deterred you from sending anything to them.

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You already have a natural inclination to be skeptical and that's good, you just need to complement that with deep research on anything you intend to throw money into.