If you see a reply in your timeline, you can see the discussion thread it comes from by just clicking on the reply. This will show the reply itself in an enlarged format, and you can see the rest of the thread by scrolling up or down.

Currently threads only show replies that have federated to your server, but there's an experimental Mastodon feature in testing which automatically fetches all replies even if they haven't federated (technical details are at https://github.com/mastodon/mastodon/pull/32615).

#FediTips