The question of whether a system is universal is in general undecidable. Using a specific mathematical axiom system such as Peano arithmetic or set theory it may also be that there is no proof that can be given. (It is straightforward to construct complicated examples where this is the case.) In practice it seems to get more difficult to prove universality when the structure of a system gets simpler. Current proofs of universality all work by showing how to emulate a known universal system. Some level of checking can be done by tracing the emulation of random initial conditions for the universal system. In the future it seems likely that automated theorem-proving methods should help in finding proofs of universality.