it's not possible for a foreign key to reference a different database.

foreign keys can only be created within the same database as the two tables exist in.

it's possible to make a function, that returns, say 0/1 if a value exists in another database, and then use THAT to enforce a check constraint.

it's a rare situation where someone puts something like that in place, but my first example would help you find those situations.

so any references can only be inferred by using, say a udf plus a check constraint, or implied by joins in cross database views, maybe.