Zero Frequency Of dc

how can we proove that the frequency of dc is zero???

In Direct current, current/voltage doesn't change with time, hence frequency (f) = zero.

Frequency = (number of changes/sec), in this case the number of times the voltage changes.
A pure DC is constant. So number of changes = 0.
0/secs = 0.
However if you have a DC with a wave form such as square wave, saw tooth or even a single phase full wave rectified unfiltered DC there will be a frequency, which will be the numbers of peaks/sec.

to experimentally verify this use a circuit having a source and capacitor in series to it

Measure the voltage across the capacitor if it varies then the source is AC but if it is stable at the input voltage it is DC supply