All manufacturers measure from the center of the bottom bracket (bb) to either the center of the top-tube (tt), or the top of the top-tube (tt). Usually in centimeters (go down to HomeDepot and they have a metric tape measure for about $8). Now, after saying that, newer manufactured frames have extended head-tubes, extended seat tubes, so how do you measure this now? For example, my SOMEC is 57cm from center of bb to center of tt, but it is 60cm from center of bb to center of the piece of seat tube that sticks out above the tt. Probably the best way is to also include the tt size - center of the head-tube to center of the seat post tube. If the tt's match, then the rest of the frame should be pretty close.

Hope this helps.