Ultralight bosons can induce superradiant instabilities in spinning black holes, tapping their rotational energy to trigger the growth of a bosonic condensate. Possible observational imprints of these boson clouds include (i) direct detection of the nearly monochromatic (resolvable or stochastic) gravitational waves emitted by the condensate, and (ii) statistically significant evidence for the formation of "holes" at large spins in the spin versus mass plane (sometimes also referred to as "Regge plane") of astrophysical black holes. In this work, we focus on the prospects of LISA and LIGO detecting or constraining scalars with mass in the range m(s) is an element of[10(-19), 10(-15)] eV and m(s) is an element of[10(-14), 10(-11)] eV, respectively. Using astrophysical models of black-hole populations calibrated to observations and black-hole perturbation theory calculations of the gravitational emission, we find that, in optimistic scenarios, LIGO could observe a stochastic background of gravitational radiation in the range m(s) is an element of[2 x 10(-13), 10(-12)] eV, and up to 10(4) resolvable events in a 4-year search if m(s) similar to 3 x 10(-13) eV. LISA could observe a stochastic background for boson masses in the range m(s) is an element of[5 x 10(-19), 5 x 10(-16)], and up to similar to 10(3) resolvable events in a 4-year search if m(s) similar to 10(-17) eV. LISA could further measure spins for black-hole binaries with component masses in the range [10(3), 10(7)] M-circle dot, which is not probed by traditional spin-measurement techniques. A statistical analysis of the spin distribution of these binaries could either rule out scalar fields in the mass range similar to[4 x 10(-18),10(-14)] eV, or measure ms with ten percent accuracy if light scalars in the mass range similar to[10(-17), 10(-13)] eV exist.