Hi Brijesh,
the easiest way is to look at the dominant configuration of the excited state and then visually inspect the MOs which take part in this excitation. Pi and Pi* orbitals are easily identified and the lone-pair type orbitals are typically perpendicular to the pi plane and have a significant contribution on the atom where your Lewis structure would predict a lone-pair. Additionally n-pi * excitations often feature a low oscillator strength, due to selection rules based on local symmetry. A typical example are for instance carbonyl compounds, which feature a very weak n-pi* excitations (in case of fomaldehyde it is only vibronically allowed) and a rather strong pi-pi* excitation.
Best regards,
Christof