We study the photoelectron momentum distribution from strong-field tunneling ionization of two molecules with similar highest occupied molecular orbitals, i.e., ${\mathrm{O}}_{2}$ and ${\mathrm{CO}}_{2}$. We find that the tiny difference between those two molecular orbitals can be clearly identified from the photoelectron momentum distributions in an elliptically polarized laser field, while it can hardly be identified in a linearly polarized laser field due to the strong Coulomb focusing effect. Furthermore, we show that the momentum distribution in the elliptically polarized laser field can be used to reveal how the molecular orbital affects the ionization time distribution of the photoelectron.