I drove through London last week. Mostly on the the A406. I only briefly saw an announcement about the congestion fee, when entering london on the M11. I didn't see any camera or any other signs on the road, that I actually entered the zone where I would need to pay a congestion fee. The question is, how do I see that I am entering a congestion fee area and is there a way to find out as a non resident (ie. no british license plates) if I have a bill running in the UK now?
According to http://www.tfl.gov.uk/roadusers/congestioncharging/ , you can call them at (UK) Tel: 0333 200 1000. I can't find a form matching your exact enquiry online. You can pay a penalty online (when you get the penalty form), or you can pay in advance, but I don't see a way to see 'Is there a penalty I owe I haven't found yet?'
Short answer, you need to have "driven by" one of these signs:
Here is the actual map: