Google Groups doesn't provide an option to be notified when someone replies to one of your topics. Furthermore, when you join a group created for a Google service, the email notifications are disabled by default. There is an option to receive notifications for individual threads, but it's very hard to find it. You have to go to thread you've just created, click on the Options link displayed next to the title and select "Email updates to me". This way, you'll be able to read the answer in your mail client, without having to go to Google Groups and find your post.
Unfortunately, the notifications are sometimes delayed, but Google doesn't provide reliable alternatives. You could manually check the posts from your profile or add an iGoogle gadget that also displays the number of new message from each thread.
