I see a lot of posts here where someone will post a picture of a bike or a frame without saying what size it is, and other people will say something like "Oh, I'd buy that if it were a little bigger/smaller," or they'll just somehow just know the size outright. How are you doing that - is it just experience?

If I'm browsing through pictures on Craigslist or eBay, I look at how much space there is between where the top and down tubes join the head tube, but other than that I have no clue how to estimate the size. Are there other tips/tricks?