In the clinical setting, preload is referred to as the end-diastolic pressure or (more precisely) end-diastolic volume
A higher central venous pressure (CVP) and / or a higher pulmonary capillary wedge pressure (PCWP) is still often regarded as an indicator of higher preload (CVP for the right heart, PCWP for the left heart). This is mainly due to the limitation that pressure cannot be directly transferred into volume. So any volumetric parameter assessing the filling of the ventricle at the end of diastole reflects the actual preload more precisely.
The Frank-Starling law states that the greater the volume of blood entering the ventricle during diastole (end-diastolic volume), the greater the volume of blood ejected during systolic contraction (stroke volume) and vice-versa. This is an adaptive mechanism of the organism to compensate for slight changes in the ventricular filling.
Gassanov N. et al. The PiCCO catheter. Dtsch med Wochenschr 2010; 135(46): 2311-2314