How do you judge if a backlink is actually worth getting?

I think the most important thing is that the backlink must be placed on a reputable website. There's a huge difference between a backlink from a reputable site and a backlink from an unknown site on the internet.

As long as the resource is somewhat relevant and credible, I am generally open to it.

Check relevance and real organic traffic first. then review the site's backlink profile, outbound inks and spam signals. DR/DA is useful but it shouldn't be the deciding factor.

Read the paragraph around the link. If the link makes sense there, it is a better sign.

My checklist :
1. How relevant is this website to your brand?
2. How old is the website?
3. How many times the website is updated?
4. How much authority that website has in it's niche?

everybody speaks about traffic but you must pay attention to the kind of traffic it really is.i noticed lots of link farms having high traffic but as soon as you look closer you see that all the traffic comes from just one page ranking for such a topic as, letsay salary of the celebrity.take care of the overall traffic but do not forget to check out which pages receive this traffic.it always good when the traffic is distributed through the whole site rather than concentrated on several pages.also check out whether the page that will host your link is indexed because sometimes people sell links from pages which are not even indexed by Google.