Hints of pre Big-Bang physics and how to detect them? The Big Bang model is the current large-scale accepted and tested theory. However, several new physics models or theories propose that there could be a previous Universe. To what extent can be test this hypothesis? I mean: are the CMB polarizations and other fingerprints like the likely cosmic neutrino background or the gravitational waves from the beginning of the Universe the only tests of pre-Big Bang theories?
P.S.: I know this question is highly speculative.
 A: Scientific theories must be falsifiable, and one pre-Big Bang theory meeting that requirement is Nikodem J. Poplawski's "Cosmology with torsion", described in numerous preprints (written between 2009 and 2020) that are available on Cornell University's Arxiv website, as well as in such well-respected journals as Physics Letters A.
His cosmology provides for the creation of local universes, each permanently separated from any other (at least until after the decoherence of any composite particles otherwise usable in any form of "clock"), that would include our own "Universe", as well as its "observable region" (the part of it which we can actually see, at least as that part appeared in its past).
His model is based on Einstein-Cartan Theory, developed through conversations between Einstein and the mathematician Elie Cartan that were completed in 1929, four or five years after the discovery that subatomic particles spin. ECT is more complex, mathematically, than Einstein's 1915 theory (General Relativity).
ECT requires that fermions have a tiny spatial extent (a little greater than the Planck length), and the cosmology is based on the interaction between virtual fermion/anti-fermion pairs and the vastly-larger fermions of any rotating star that's collapsing gravitationally (after the depletion of its nuclear fuel has left it without the radiation pressure needed to prevent that collapse).  The "Event Horizon" of the star's collapse, propagating outward from its center, separates the virtual particles of many fermion/anti-fermion pairs, thereby preventing them from annihilating each other, with the one remaining on the inboard side of that horizon being materialized by the mass/energy equivalence of the star's increasingly concentrated gravitational field.
When the newly-materialized fermions come into contact with the vastly larger ones of the star itself, the spin of those stellar fermions propels many of the newer ones outward into the outer reaches of the collapsing star's original volume, where, as their momentum decreases, they form a new "local universe".
In a 2015 paper that can be seen at https://arxiv.org/abs/1510.08834, Desai describes the consistency of Poplawski's "local universe" with our Cosmic Microwave Background radiation, without any need for the specialized "inflaton" particles whose identification has become problematic for inflationary theories relying on a hypothesized scalar field.
His theory would also appear to have some potential for explaining the huge disparity (at 121 orders of magnitude) between the observed and theoretically-hypothesized amounts of vacuum energy, as it would provide for a temporal multiverse endlessly replicating its temporally "local" universes on sequentially-decreasing spatio-temporal scales.  (Due to quantum uncertainty, the replications would only rarely be exact.)
Because passage in either direction between the parenting and parented universes of each sequence would be prevented by the insufficient total energy and extreme gravitational distortion of time on the "parented" side of the EH, no evidence of either would be present in the other, aside from the indirect evidence for the existence of black holes of stellar origin.  (Most such evidence consists of the elliptical orbits still followed by stars whose binary partner has collapsed into a black hole, since most stars are in binary pairs.)
Poplawski's model is potentially eternal both to the future and to the past, and is potentially falsifiable because it depends, for each of the local universes of his temporal multiverse, on the rotation of its parenting star, which is "inherited" from its parenting LU.  Most stars rotate appreciably, and all stars have generally been expected to have at least some faint residual rotation, but Poplawski's theory might reasonably be considered less than probable if the observable stars did not, overall, have a prevalent direction of rotation.
In fact, such a prevalence would probably be minimal, as studies on the subject practically alternate between favoring such a prevalence and discrediting it:  The most recent I have found, at
https://hal.archives-ouvertes.fr/hal-02045149v3/document, dates from January of 2020, and favors that prevalence.
A: Cmb and all other methods you mentioned above are not for discovering pre big bang physics at all. They are for understanding the event of the big bang, so... post big bang not pre. Unfortunately there are no known ways (not even theoretical) to test whether or not another universe preceded our own. Besides I seriously doubt there would be any detectable remnants of that universe and how would we distinguish what was from ours and what was from the other hypothetical universe. 
