I'm in a similar boat as you - from what I can tell, best bet is to buy from Chrono24, used, from a dealer with good reviews, and pay ~250 additional for chrono24's certification service in which they guarantee its authenticity.

I've been waiting for years to pull the trigger on the Speedmaster, and recently decided on the white dial one. I say decided, but mean that's the one I want but can't bring myself to click purchase :]