If site A links to site B and B in return links back to A it can be considered as a reciprocal link exchanging. And it says that recips are less effective comparing to one way links. My question is, how do SEs identify reciprocal links? Once they found a link to B from A, do they crawl B all over to see whether B links back to A?
Just because site A links to site B and site B links to site A doesn't mean that the links will be devalued. If the links are both on links pages then they will have virtually no weight. But imagine if the links are both from pages on the site such as a blog. You can then link to a site and they can link back and the links still pass weight and are not considered reciprocal links.
Search engines keep track of links going to and from a page. It doesn't take much for them to look that up in the database. They don't have to recrawl "site B" every time they find a link to it. Reciprocal links are also easy to detect because they generally appear at the same time.