Active matter usually reaches a stationary state but the steady state distribution is different from the Boltzmann one and can not be, in general, described by a single temperature. To define an effective temperature in a system of self propelled particles, we consider the parameter that replaces the bath temperature in the fluctuation-dissipation relations between the time-delayed correlations and linear responses of the same observables. It can be shown that there exists an effective temperature taking the same constant value for all observables that evolve in the same time scale and that this quantity plays the same role as the equilibrium thermodynamic temperature (for example it controls heat flows). We focus on its dependence on the global density and on the activity in a system of spherical and elongated active Brownian particles. To measure the linear response we exploit recently developed methods that do not imply the application of a perturbation of vanishing strength. Firstly, with the help of molecular dynamics simulations, we study the behavior of the effective temperature in homogeneous and we found that the effective temperature decreases monotonously with increasing density. Secondly, we investigate the temperature behavior when the system undergoes motility induced phase separation and coarsening.