Is there a way to demonstrate that the problem of determining whether a given path exists in a graph is NP-complete?

Yes, the problem of determining whether a given path exists in a graph can be demonstrated as NP-complete by reducing it to a known NP-complete problem, such as the Hamiltonian path problem. This reduction shows that the path existence problem is at least as hard as the known NP-complete problem, making it NP-complete as well.