We present high-, mid-, and low-resolution spectroscopy of the field brown dwarf candidate PC 0025+0447 (M9.5) spanning a 4 yr baseline (1994-1998). The strength of the emission lines and the amount of optical veiling are very variable. Our spectra taken at an epoch of low veiling allow us to detect the lithium resonance line, which was not detected when the veiling was high. The presence of lithium proves that PC 0025+0447 is a substellar object less massive than ~0.06 M_solar and younger than ~1 Gyr. We also present mid-resolution spectra of three brown dwarf candidates in star-forming regions, namely rho Oph 162349.8-242601 (M8.5), V410 Tau X3 (M6.5), and V410 Tau X6 (M6). Lithium is detected in all of them, supporting their very young age. Of these three objects, only the one in rho Oph is sufficiently cool to warrant an unambiguous substellar status. The spectroscopic characteristics that PC 0025+0447 and rho Oph 162349.8-242601 have in common are moderately strong Li I lines (pseudo-equivalent width ~1 Å), persistent Hα emission with equivalent width greater than 50 Å, and weaker K I and Na I lines than field dwarfs of the same spectral type.