  The photoevaporation model is one of the leading explanations for the
evolution of small, close-in planets and the origin of the radius-valley.
However, without planet mass measurements, it is challenging to test the
photoevaporation scenario. Even if masses are available for individual planets,
the host star's unknown EUV/X-ray history makes it difficult to assess the role
of photoevaporation. We show that systems with multiple transiting planets are
the best in which to rigorously test the photoevaporation model. By scaling one
planet to another in a multi-transiting system, the host star's uncertain
EUV/X-ray history can be negated. By focusing on systems that contain planets
that straddle the radius-valley, one can estimate the minimum-masses of planets
above the radius-valley (and thus are assumed to have retained a voluminous
hydrogen/helium envelope). This minimum-mass is estimated by assuming that the
planet below the radius-valley entirely lost its initial hydrogen/helium
envelope, then calculating how massive any planet above the valley needs to be
to retain its envelope. We apply this method to 104 planets above the radius
gap in 73 systems for which precise enough radii measurements are available. We
find excellent agreement with the photoevaporation model. Only two planets
(Kepler - 100c & 142c) appear to be inconsistent, suggesting they had a
different formation history or followed a different evolutionary pathway to the
bulk of the population. Our method can be used to identify TESS systems that
warrant radial-velocity follow-up to further test the photoevaporation model.
