Checking river levels online is an effective way to discover what the river will be like before leaving home.
Natural Resources Wales, unusually, provide an excellent online service in this respect, you can access up to date river information by visiting -
Zoom into the map and choose whichever local monitoring station you're interested in.
I tend to use the Pen-y-bont ar Ogwr monitoring station as a rule. In my experience when the level there drops to 0.600 metres it's getting about right for the fly. You may find that doesn't suit you but there you go, do set your own parameters, the main thing is to keep safe and choose a river level which you feel comfortable with.
Experiment with the drop-down menus and you get a few choices of view. I like the month view, it puts it into a more meaningful perspective. After the last five months' weather what's the betting there'll be a flood on opening day...