To determine peak voltage from an oscilloscope trace, the maximum vertical deviation of the trace from the centre line is measured from the screen.
This number of divisions is multiplied by the Y gain from the oscilloscope controls.
In the example above, the red line shows the number of divisions to be measured - 4 divisions for this trace.
However this is not the effective voltage of the trace. A more useful measure of a.c. voltage is the r.m.s. or 'root mean square' voltage.