Common things to check for both are
- the backlink profile, a good number of HQ genuine referring domains/links (ideally 150+)
- any sudden drops/spikes in traffic that might refer to a penalty
- regularly posted content, it should be at least a post per week
- spam-free with a limited number of OBLs, not some spammed to death website/PBN
- friendly design & genuine-looking website
- decent published content
- good on-page, silo & some internal linking
- niche & language relevant if it's possible, that would make a better impact
When it comes to guest posts, you should check the website traffic & top-ranking keywords along with their difficulty,
a good website should at least be ranking for a couple of competitive keywords,
not just a website targeting easy useless keywords for the sake of selling links
Not the case for a PBN tho, most of the PBNs do not have any traffic since they're barely made for the sake of passing link juice & authority,
Along with all of these, you should check PBN footprints (design, categories, registrars, hosting, URL structure, etc...), domain(s) history, OBL (especially for PBNs),
Getting homepage links would make a better impact as well, it's harder to acquire with guest posting websites but commonly sold by good PBN sellers