It's a question I see asked a lot in reviews, and kudos to the hobbyists asking – it is very important.
If a provider isn't screening, she leaves herself wide-open to anything, including LE. Just because a provider is legitimate today doesn't mean that's who you'll be having a date with tomorrow, in the event that she's compromised.
So how can you be sure?
I advise my clients that provide me as a reference to check back with me before following through with the appointment. Send me a quick text or PM, and ask if she's checked with me.
I'm sure that other providers of quality wouldn't mind giving you the same courtesy. Taking measures to keep yourself safe also keeps us safe.
If she hasn't checked, you'll know she's not screening.
And if she hasn't, and you see her anyway –– she's not the only person leaving herself wide-open.
Just my $.02.