A possible mechanism for setting the dark matter (DM) relic abundance is freeze-in, where the DM is produced by extremely feeble annihilations of Standard Model particles. Despite the very small couplings of the DM, if the annihilation occurs via a light mediator then this is a predictive scenario with implications for direct detection and cosmology. I will present ideas and prospects for testing sub-GeV freeze-in candidates with new direct detection targets, the CMB, large scale structure, and other observables.