I sent this as a reply to a question on another topic, but then thought I'd post it here separately in case it may help anyone.
You can find out whether your tree is marked as a "public tree" by clicking on your user name at the top of the ancestry screen; go to "Member Profile" and scroll down; if your tree is a public one it will be shown as public. On that same page, you can click on the box "switch to public view" to see what other members can see about you.
If by chance you thought your tree was public but see that it's private, you can go to the top bar and click on Family Trees, then click on the tree you have linked to your DNA test. You can then click "tree pages" and view the Privacy Settings page, where you can select your tree to be either public or private.
To be sure you've remembered to link your DNA to your family tree, go to your DNA home page and click on the name of the person who took the test; then click the "wheel" to the right of the name and you see the test settings page. Be sure you have the test linked to the tree. As far as I know, you can have as many DNA tests as you want, but you cannot link them to more then one tree, unless you have purchased several memberships!
I often wonder, when I see so many of our matches with the tester having no tree or being listed as a private tree, if they just didn't know how to set it up. While I understand that some people take the DNA test for reasons other than genealogy, it does seem a shame to not be curious enough to find out if you have family connections through this site.