Does the energy signal or finite energy signal have 0 dc component always?How to confirm this from the frequency spectrum?
An energy signal $f(t)$ satisfies
i.e. it has finite energy. If $f(t)$ had a non-zero DC component $f_0$ defined by
the integral in (1) wouldn't converge. So an energy signal must have a zero DC component.
Note that there is some confusion about the term "DC component". Some people would call $F(0)$, i.e. the value of the Fourier transform of $f(t)$ at frequency $\omega=0$, the "DC component" of $f(t)$. However, note that $F(0)$ given by
can be non-zero (but finite), even if (1) is satisfied. One example of such a signal satisfying (1) but with $F(0)\neq 0$ is a sinc pulse.
Also have a look a this answer to a related question.