We present a flexible method to study the optical properties of an Er3+ ensemble in Si accessed via resonant excitation and in situ single-photon detection. The technique allows an efficient resonant photoluminescence detection of optically active centers with weak oscillator strength in transparent crystals without the need for nanofabrication on the sample. We observe 70 Er3+ resonances in Si, of which 62 resonances have not been observed in the literature, with optical lifetimes ranging from 0.5 to 1.5 ms. We observe inhomogeneous broadening of less than 400 MHz and an upper bound on the homogeneous linewidth of 0.75 and 1.4 MHz for two separate resonances. These narrow, stable resonances confirm Er3+ in Si as a promising quantum information candidate. We discuss the use of this technique for rapid characterization of future samples, with the aim of enhancing the prevalence of sites which are favorable for quantum information applications.