Question

how we can say a circuit is valid or not

Answer 1

If you model the circuit and id does not perform, it is not valid. You can model the circuit manually using math or you can enter the circuit into simulation software such as SPICE Qucs - Transistor models - Verilog - VHDL - Yenka - PLECS - or Livewire_(software) and see what the perimeters and function is.

If you are not in a lab at MIT or some other school you may want to use the simulators provided by Free Open University or some other Collegiate Independent Study resource providing such.