In reply to crikeyorikey: A quite often get calls asking me this sort of question, and a can give advice, but the BMC does not dictate policy to climbing walls. The walls must make their own policy which depneds on how the wall is run, designed and managed and what is dictated to them by their own insurance.
I definitely recommend some sort of comeptency testing, and some walls will find that a quick questioning at the front desk is usually good enough to decide that the climber knows what they are doing or not.
However some walls prefer to test the climber by some sort of demonstration. Like I said this is down to the wall and how they manage themselves. Doing individual tests might be prohibitive in a really busy wall as there just wound not be enough time to test everyone, however they have the advantage of lots of people being around to spot other mistakes before an accident happens.
Smaller walls might not be as busy and a mistake may go unnoticed, so they may prefer to test to make sure all their users are competent. There is nothing wrong with having a chat with someone at the wall you are in if you think they are doing something dangerous, as you could save their life.
There seems to be this stigma in walls against apporaching someone, just in case you get told to bugger off, but if you think an accident may happen please do intervene. I do it all the time, and I am met with a variety of reactions, but I am sure I have prevented a couple of accidents.
Cheers