I have seen a test done on the internet that looks quite good.
Fill your cleaner with just water, you then arrange three pieces of aluminium foil in the cleaner, one at each end and one in the middle. Let the cleaner run for three minutes. when it has finished you should see the foil has small holes in it, maybe even big holes but from what I have been reading the cleaner is most effective in the middle of the tank. If you withdraw the foil and it looks the same as it did when it went in this is probably a sign its not working. Dont be suprised also to find the foil pieces you place at the ends are not as affected as the centre one, this seems to be standard.