The HasConstant( descriptor ) command returns true if descriptor is the full name or symbol of a physical constant in the ScientificConstants package.  Otherwise, it returns false.

$\mathrm{with}\left(\mathrm{ScientificConstants}\right)\:$

$\mathrm{HasConstant}\left(c\right)$

$\mathrm{true}$

$\mathrm{HasConstant}\left(\mathrm{speed\_of\_light\_in\_vacuum}\right)$

$\mathrm{true}$