You make a guess for the molecular geometry, fire up your favorite quantum chemsitry program, and tell it to optimize the geometry and calculate the dipole moment using some electronic structure method (e.g., Kohn-Sham DFT).
To cgk,
sir I know the geometry of the molecule is trigonal bipyramidal. I just need the value of dipole moment for the molecule V2O5 (Vanadium Pentoxide).