For me a thread that I have read is no longer high lighted like the ones I have not read. If you just click your back button then you would need to refresh the screen. Any new comments would cause it to be bolded again. That is how I tell. If you scroll up and click the back button preps has I don't think you would have to refresh it.
The menu button on the top left of screen, click on it, scroll to bottom, and there is a "mark forums read" button. Just click on it and it will take you to another screen to actually mark them read.