A factor is a rule for ranking securities. Momentum ranks by trailing return; low volatility ranks by trailing realized volatility. The rule itself is public knowledge. What matters is whether the ranking is built correctly and whether it has done, out of sample and after costs, what the literature says it should.
Marnello applies three tests before a factor is presented as a source of return. A Fama-MacBeth cross-sectional regression has to show a risk premium with a t-statistic above 2. Decile portfolios have to show a positive information ratio in a holdout period that was not used to build the factor. And the top-minus-bottom decile spread has to remain positive after 20 basis points per side of transaction cost. Separately, where a public counterpart exists (momentum and size), the in-house series is compared with the corresponding series from the Kenneth R. French Data Library, so that anyone can check the construction independently.
A factor that fails is still computed and reported, because a portfolio's exposure to it is a fact about that portfolio whether or not the factor earns a premium. It is simply labeled as a risk descriptor rather than a return source. In the current library, 1 of 4 families meets the rule on the free-data universe; the validation record says so.