Question

is there any rule that you are supposed to use when determining whether its in ground state or excited state

Answer 1

excited state is normally short lived, you can assume most of the time that it is in the ground state which is the lowest energy. I'm pretty sure excited state atoms or molecules are denoted with an astrix (*) but then again I cannot really recall.

Answer 2

Experimentally, that's true what is stated above. But theoretically, you should look at the molecular formula given to determine this.