Do you mean an Ancestry Member Tree, on line at this site? If so, do a search, specifying that you want to search member trees, for the home person's name. So far as I know, it isn't possible to search by tree name - but if you search for the person in the trees database, you can then look for that tree name among the possibilities.
A couple of things to be aware of - first, the site rarely is indexed now, so if the person (or entire tree) was added in the last six months or so, you may not get a hit in your search returns, even if it is there. Second, if the tree is private, you will need the owner's permission to see it, although you still should be able to find the person through the method I described.