If I were to do it seriously (as opposed to passing the time as I cycle to work), I think I would have to pick a nice big junction and spend an hour every morning and evening counting the cyclists.
I often see some people at the Chelsea Bridge / Chelsea Embankment crossroads who must be doing some kind of traffic monitoring. Hi-viz jackets (it must be dangerous standing on the pavement) and clipboards but no idea what they're actually looking out for. I'll stop and ask if ICBA next time.
And RLJ rates will be different at different junctions. Not many people try and jump the lights on Millbank/Grosvenor Road crossing Vauxhall Bridge Road, but they all stream through the lights by the South end of Royal Hospital Road or Lupus St.