The specific heat C of superconductors is strongly depressed just below the transition temperature by an increase of the magnetic field ΔB. A measurement of the function |∂(C/T)/∂B| for different magnetic fields at T≈Tc, which can be approximated by a Lorentzian of half-height width ΔT(B)αBn, is found to be a direct way of determining the dimensionality of a superconducting system in the LLL (lowest Landau level) approximation, with n=1/2 for 2D and n=2/3 for 3D. No baseline subtraction is required. Examples are given using high resolution specific heat measurements between 0 and 16 tesla on YBa2Cu3O7−δ (YBCO) and Bi2Sr2CaCu2O8 (BSCCO) single crystals. The temperature TcB(B) of the minimum of |∂(C/T)/∂B| can be related to the mean-field critical temperature Tc(B) in the framework of the theory of Tešanović at high fields.