If you're viewing a page showing comments which are down a comment tree and don't have their context shown at all, there should be an explanation of that fact above the shown comments explaining that you're viewing comments without their full context. There should be links to the full context (the top of the tree).
Maybe it should say:
You are viewing a comment thread without its full context.
View the full context [link to the top of that comment tree]
View all the comments [link to the post/all comments]
If you're viewing a page showing a portion of comments starting at a parent level comment, it should probably say:
You are viewing a single comment thread.
View all the comments [link to the post/all comments]
If you're viewing a page showing comments which are down a comment tree and don't have their context shown at all, there should be an explanation of that fact above the shown comments explaining that you're viewing comments without their full context. There should be links to the full context (the top of the tree).
Maybe it should say:
If you're viewing a page showing a portion of comments starting at a parent level comment, it should probably say: