True Random Number Generators (TRNGs) are sensitive Intellectual Property (IP) blocks involved in the creation of cryptographic keys, initialization vectors, nonces, etc. They must behave properly within a large environmental spectrum, including multiple corners, in case of aging-induced change of device characteristics over time, and also under intentional attacks aiming at lowering the TRNGs entropy. In this paper, we review normative and technical landscapes in this respect, and propose a pre-silicon verification methodology to assess the resilience of TRNGs. In particular, we qualify the unitary free running oscillator (FRO) entropy source analytically, and then extend the study to a full FRO-based IP module. Our results encompass analytical characterizations in terms of jitter measurements and certification-based characterizations in terms of tests.