There are a couple of things you can do. But first you have to understand that PR is still getting updated, just not to the public. Backlinks create link juice and that link juice is actually pagerank so google are still using PR it's just not displayed to the public anymore to avoid spam.
1. Social Media
A few things you can do to determine if a site is an authority site or not is to look at their social media accounts. If they're getting a lot of engagement then you know their site is probably getting shared all over the net. So you also have to look at social shares on their social profiles and blog.
Shares attract a lot of organic backlinks and increase a website's authority. So look at their social media and blog social shares.
2. Blog Engagement
Another thing to look at is the blog comments, check if they get a lot of engagement on their blog as well. Check at least 10 of their blog post and see if people are always engaging with their content or not. If they have a good engagement on each blog post then they probably are an authority site.
3. Check Traffic
Use a tool like SEMrush to check their blog traffic, check their seo, see if they get paid traffic and an estimation of how many visitors they get in a month. Also check if they're monthly traffic is growing of decreasing. Traffic is another good way to see if a site is an authority site or not.
4. Check for backlinks
Use sites such as moz, majestic and ahrefs to analyze the backlinks for each site. I also look at the anchor text to make sure it's not spammy and check the sites that are linking to them. For instance if I'm analyzing BLOG 1 and I find that BLOG 22 has links to BLOG 1 then I also have to check if BLOG 22 is also an authority site.
You have to make sure that the site you're targeting has some good links from other authority sites as well.
I don't care about things such as trust flow, citation flow, domain and page authority (DA/PA). Those don't mean squat to me, so I don't look at them.
Hope that helps.
