While you trade link with another site you must pay attention specially to one thing to avoid to be cheated. There are lot of webmaster that try to take advantage of the trade, taking backlink from you, without giving one back to you, and you can’t know if this happens if you don’t examine HTML code of the page where your link should be present. There are two most diffused technique to do this cheat.
There are webmaster who put in the head code of the page meta tag noindex for robots. This tag “forbids” to robot to elaborate the page. In this mode, when robot/spider of some search engine visits this page, he doesn’t index it, and you haven’t the link to your sites. The head meta tag mentioned is wrote like this: <meta name="robots" content="noindex"> .
Other webmaster instead use attribute nofollow in the anchor tag. This attribute says to search engine’s robots not to follow the link at which the attribute is related, and even here no backlink for your site. An example of this technique is this
<a href="http://www.mysite.com" rel="nofollow">Link to my site </a>
This concept can be done using meta tag, too, inserting the attribute nofollow. It’s less used way while all links on the page are not followed, including the inner links of the same site, and this is disadvantage for the webmaster who apply this method
When I exchange link with somebody, I always control inner code of page to see if there are some of this tags, and I make other 2 controls: I control if the page with my link is accessible from the main page of the site (often non honest webmaster hide the page from the rest of the site), and if the page is present in the Google cash: if not, maybe robot.txt prevent the page from scanning, so, in this case, I control the robot.txt file, too.