If you had two databases, that had two tables between them that would normally implement a one to one (or many to many) constraint but cannot since they are separate databases, how would you validate this relationship in an application or test?

Is there a simple way to do this? For example, a tool or technique that can, given a constraint type, tables and fields, does the validation.

I imagine that this isn't the first time this come up so I'm hoping people can share their solution.

Thanks.