IMO, most of the PBN providers are here at BHW marketplace. Just check the below:
1. Quality of the domains
The general metrics you should look into: PR, DA/PA, AGE. Also try to figure out if they are serious about using good domains that have clean backlink profiles, not those spammed domains with amazing metrics, as they will pass the penalties to your site.
2. Footprint
Are they trying their absolute best to run the network as private as possible? Are the network of sites all hosted on unique c-class ips? Are they using any measures to protect the privacy of the domains? Try to find the one provider that uses tons of mix in everything.
3. Content
Are they using unique content for EVERY POST? This is one of the most important factors IMO. As it's not just about avoiding penalty, it's just a total waste of time to use spun content that would be ignored by G. Also, will they dripfeed your posts? Don't choose a provider that will just blast your posts in one day.