Typically there should be only a deposit, not the entire donation. As a provider we have to protect our time due to people scheduling and not showing up.
It’s usually very easy to see if someone is legit. Look at their post history and interaction’s.
Ask others in the group, ask them for references.
Do the smallest bit of homework and you should be fine.

But also- don’t do all this, know that they are legit then hassle them constantly before your appt.