  The non-thermal production of dark matter (DM) usually requires very tiny
couplings of the dark sector with the visible sector and therefore is
notoriously challenging to hunt in laboratory experiments. Here we propose a
novel pathway to test such a production in the context of a non-standard
cosmological history, using both gravitational wave (GW) and laboratory
searches. We investigate the formation of DM from the decay of a scalar field
that we dub as the reheaton, as it also reheats the Universe when it decays. We
consider the possibility that the Universe undergoes a phase %of kination with
\textit{kination-like} stiff equation-of-state ($w_{\rm kin}>1/3$) before the
reheaton dominates the energy density of the Universe and eventually decays
into Standard Model and DM particles. We then study how first-order tensor
perturbations generated during inflation, the amplitude of which may get
amplified during the kination era and lead to detectable GW signals. Demanding
that the reheaton produces the observed DM relic density, we show that the
reheaton's lifetime and branching fractions are dictated by the cosmological
scenario. In particular, we show that it is long-lived and can be searched by
various experiments such as DUNE, FASER, FASER-II, MATHUSLA, SHiP, etc. We also
identify the parameter space which leads to complementary observables for GW
detectors such as LISA and u-DECIGO. In particular we find that a kination-like
period with an equation-of-state parameter $w_{\rm kin}\approx 0.5$ and a
reheaton mass $\mathcal O(0.5-5)$ GeV and a DM mass of $\mathcal O (10-100)$
keV may lead to sizeable imprints in both kinds of searches.
