take a measuring tape or ruler.... put the end at the center of the crank spindle.... then measure to the end of the seat tube...
Now is where it gets tricky.... some manufacturers measure from center of bottom bracket to center of top tube. if you measure this way, you measure up to approximately center of where the top tube meets the seat tube.
Other manufacturers measure from center of bottom bracket to top of top tube. if you measure this way, you measure up to where the top of the top tube meets the seat tube.