T5B is generally open, T5C is generally closed, at the time of writing. In the case of T5C, as you move towards the centre of the building after the airbridge there is usually a big sign up saying T5C is closed and thus go to T5B. For T5B it's pretty much en-route anyway, just down one level and it is signposted, so in the rather unlikely event that is closed you would only lose a few seconds.