How can understand periodicity of a Signal from frequency domain representation? Is it possible to say a signal is periodic from its frequency domain representation? A periodic signal is sum of its sinus and cosinus. Frequency translation of sinus and cosinus functions are symmetrical impulses. Is it correct to say "a signal is periodic if its frequency domain representation includes symmetrical inpulses"?
A signal is periodic if and only if its frequency spectrum is


discrete (contains only pulses covering a single locus with a finite
area).
all of those discrete pulses occur at an integral multiple of one
frequency (the periodicity).


If you have, for example, $\cos(\pi t)+ \cos(t)$, the resulting spectrum is discrete but the function is not periodic.