We describe a spectroscopic test capable of providing direct confirmation of the substellar nature of brown dwarf candidates. While the conditions for substantial lithium burning are reached even in very low-mass stars, in substellar objects with less than 0.06 solar mass such conditions are never attained and Li is expected to be preserved. Therefore, the detection of lithium in the spectrum of a brown dwarf candidate would be of great interest to confirm its substellar mass. We report on a search for the Li I 670.8 nm resonance doublet in some very low-mass dwarfs and brown dwarf candidates, namely, GL 65 (A and B), GL 406, GL 234AB, GL 569B, and GL 473AB. We do not detect the lithium feature in any of the spectra of this work. This implies that the objects in our sample have suffered from heavy Li depletion (more than four orders of magnitude in most cases). These objects consequently present masses higher than 0.06 solar mass and appear to be older than about 10 exp 8 yr.