Lubotzky's theorem is a necessary and sufficient set of conditions for a finitely generated discrete group to be linear, i.e. isomorphic to a subgroup of $GL\_n(K)$, where $K$ is a field of characteristic 0. Its proof relies on the (relatively) advanced theory of pro-$p$-groups. It can be found with its proof in the book, "Analytic pro-$p$-groups" of Dixon, Du Sautoy, Mann and Segal, Interlude B, or in the original paper "A Group-Theoretic characterization of linear groups" in Journal of Algebra 113. 


My question is:



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> Has this criterion being used to prove or disprove the linearity of discrete groups of independent interest?
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By independent interest I mean some group interesting for mathematicians outside of the theory of pro-$p$-groups, for instance the Braid groups $B\_n$, the group $Out(F\_n)$, fundamental groups of interesting manifolds, etc...
not some group 



Applications of Lubotzky's linearity theorem?
According to Alex himself, this theorem is practically useless. It does not mean that it can't be applied, for instance when you have a group with assumptions that it has many quotients in some suitable sense, it can be applied. But for explicit examples of groups (e.g., given by a presentation, or as groups of automorphisms of some reasonable structure) as in the examples you provide, you don't have much info about finite quotients so you it's not practical: in examples known to be linear, the easiest way to check Lubotzky's criterion is to show that they are linear...


