This article is worrying:

http://news.bbc.co.uk/1/hi/england/london/4361286.stm

I would like to be able to buy an Oyster reader (similar to that used
by inspectors) so I can check that my Oyster has been processed
correctly after boarding a bus or tram.

The problem is you have no proof that you have paid - at least if I had
an Oyster reader if I see a problem with my card I can try to validate
my Oyster Card again.