Imo there is no "congratulations, you wrote a merit"-sign ore something like that. So the merit counter on your userpage is the only way to see that you get one (but not for which comment you get that one)
Actually, there is a setting that allows an e-mail notification for merits.
Email will have a link to your merit. Or you can just look at your counter, seeing as most people write one or two merit worthy comments at a go, it shouldn't be too hard to notice when you got the merit and then search the games you reviewed recently.
