Question

how can you provide that carbon present in coal

Answer 1

We can get the carbon in the form of coke (which has high carbon content)
by  the fractional distillation (processing) of coal.
During the processing of coal, there are four results:-
1) Coke
2) Coal tar
3) Coal gas
4) some other volatile(combustible) substances which have high in carbon     content.
  
   Thank you! Have a nice day!
--------EDIT---------------
We can prove that coal contains carbon again from processing.
We get coke which has high content of carbon which proves that coal also has carbon.
Also, coal was formed from the vegetation which contains carbon.