Verification

How does everyone verify a potential new provider that has no reviews or has not been established?

You don't .

The only way that is a good idea is if the new provider is in an agency or its referred to you by a highly established provider

Providers don't verify with clients
We verify with agencies and or advertising sites

You clearly have experience, but I'll weigh in. I communicate with them and go on instinct. Although if they have feedback here it shows they are legit. Doesn't mean you will like them or they will show up etc. but at least are a real provider.

If looking on SipSap those are 90% scammers right now. Most of the real ones are not what you would want, so basically stay off that site. Also lot of fakes on Skipthegames and whatever other sites there are.

It is tough without reviews. As an alternative, the olden day way of word of mouth is an option. Sometimes reaching out to fellow hobbyists in the area and asking about them can help. Beyond that, you research your ass off and like the previous guy said you go by instinct. There is no substitute for experience when going off instinct.

Sometimes you can just feel it's a bad scene. Things like tone of voice. Super willing over the phone without any hesitation is usually a give away on a scam or a leo alert. If I can't find anything out there on them I just put them in the don't see until info starts popping up.