You’ve just found a topic you really want to contribute to, or someone said something and you don’t want to quote their entire post to make sure they know what it is you’re responding to, but there are so many reply buttons!
If you’re on a computer, you can actually hover your cursor over the reply button and it will tell you whether it will be to the individual post or the topic as a whole (a notice that, unfortunately, disappears as soon as you hit the buttons to take a screenshot.)
The other way to tell is location.
The reply button for the entire topic is at the very bottom of the thread. When you click it, it will also say the name of the topic at the top of the reply box:
The reply button for an individual post is inside the separating lines and in line with the like button. Instead of the name of the topic, it will show the user picture for the person you’re replying to.