Click on your name at the top of the page it will take you to your profile which will show which topics you commented on and if there have been any comments there since.
Not always strictly true. When you reply to a post, it takes time for your comment to appear. If its a slow modding day there can be several hours before it is approved and there is often a few posts that go thru on that thread together, all approved at the same time – if this is the case then your profile still shows no replies until the next time posts are approved, even though there could often be replies to earlier posts that you have not seen due to their being approved at the same time as yours. Does that make sense?