You link to a page because you think that it will help people reading your site to have the option of following that link. Does it go to a useful site that is relevant to the content of your page? If so, link to it. If not, don't.
If you mean "how do I tell if a page is worth trying to get a link from?", that's a different question, but the answer is pretty much the same. If the page is one where you think your site will be relevant and useful to its readers, try to get a link. If it's a site that is doing really well in the search rankings and looks like it's getting lots of traffic, try harder to get a link. If it's languishing somewhere around page 23 on Google and there's no evidence of it having a lot of visitors, it isn't worth putting as much effort in.