Rare-earth frustrated systems can present exotic physical properties. The newly discovered TmMgGaO4 shares the same structure as the quantum spin liquid candidate YbMgGaO4 with layered triangular lattice. Due to strong spin orbital coupling and crystalline electric field, its ground state is a singlet, a non-Kramers doublet or a quasi-doublet composed of two singlets. Different ground states will have different spin Hamiltonians. For example, Ising spin state has been proposed for the quasi-doublet scenario which, however, is inconsistent with the susceptibility measurement. Here we propose to use inelastic neutron scattering to study the crystalline field of TmMgGaO4 so that the ground state can be determined which can help us decide the spin Hamiltonian.