Knowing what size bike to look for I am presently riding a hybrid that apparently is a bit too small for me, and I'd like to replace it with a road bike. Having ridden this improperly-sized bike for as long as I have (about 8 months, including one MS150), I am interested in making sure that the next bike I buy is properly sized.

The sizing numbers published for different bike models are only somewhat helpful, because a 54cm bike, even of the same type, will fit different people depending on who made the bike. It would have been more helpful if the sizing was listed as a range of heights for riders (such as, "best fits riders from 175 to 185 cm").

Somewhat complicating the issue is that (as I have mentioned in another question) I am chronically short of funds, and there is a high chance that I'll be buying something off of Craig's List or some other such place. I don't want to waste the seller's time by trying the bike if there's any chance that I won't buy it. I'd much prefer to know when I show up to make the purchase that the bike is manufactured for a rider of my height.

The same goes for mail-order. If I go this route, I want a reasonable certainty that the bike is made for someone my size. I don't want to deal with the hassle of returning a bike that doesn't hit my own personal Goldilocks zone.

I'd also rather not trouble bike shop personnel to let me try a bike to see if the sizing is right unless I'm ready to buy from that shop (possible but by all means not certain); I like to think that I'm a golden rule sort of person and using their time in this way seems too much like taking advantage of them. However, if this is considered by them to be a cost of doing business, then it's not a show-stopper.

That all being said, what's the best way to go about finding what size bike will fit me best? (If it helps to be more specific, I'm mostly interested in a road bike.)
While Michaels answer is the more correct one, I found ETT or effective top tube length quite useful for me when buying second hand. It is easier for the seller to measure, and was more useful to me than the seat post height since that is trivially adjustable and my upper body is smaller than it should be for my height.. 

Edit: The effective top tube length is the horizontal distance from the top tube/head tube junction to the seat tube. This measurement has become significant as more and more bikes are designed with sloping top tubes. (from https://www.bikecad.ca/effective_top_tube_length)