If you are looking at the list of threads in a sub-forum, there is a small, specific icon right-justified to the right of each heading indicating if you have "subscribed" to a thread, which by default happens when you post to a thread unless you have changed the default (see attached).
As others have specified, your User Control Panel (User CP) will also list all threads you have subscribed to which have received new replies. It's actually my default bookmark for MobileRead and is the same for everyone:
https://www.mobileread.com/forums/usercp.php
If you want to look at all threads to which you've subscribed, whether they have new posts or not, there is a link down the left of your User Control Panel called "List Subscriptions". You can go straight there with this link:
https://www.mobileread.com/forums/sub...ewsubscription
Note that you can also get to that point from the "Quick Links" pull-down meny at the top of every forum page (the item "Subscribed Threads").
If you wish to change the default behaviour for what happens when you post to a thread (whether it automatically subscribes you, sends you an email, et cetera), you can find that under "Edit Options" in your User CP. A direct link is here:
https://www.mobileread.com/forums/pro...do=editoptions
(It's under "Messaging & Notification", and second sub-box down, called "Default Thread Subscription Mode").
Cheers,
Marc