CREATE DATABASE test1
GO

USE test1;
GO

CREATE TABLE tableA (
     PK int PRIMARY KEY CLUSTERED
    ,someColumn varchar(50)
)
GO

CREATE DATABASE test2
GO

USE test2;
GO

CREATE TABLE tableB (
     PK int PRIMARY KEY CLUSTERED
    ,aForeignKey int NULL FOREIGN KEY REFERENCES test1.dbo.tableA(PK)
)
GO

Msg 1763, Level 16, State 0, Line 19
Cross-database foreign key references are not supported. Foreign key 'test1.dbo.tableA'.
Msg 1750, Level 16, State 0, Line 19
Could not create constraint. See previous errors.

CREATE TRIGGER TR_Fake_FK ON dbo.tableB
    FOR INSERT, UPDATE
AS
BEGIN
    SET NOCOUNT ON;

    IF EXISTS (
        SELECT *
        FROM INSERTED AS I
        WHERE NOT EXISTS (
            SELECT *
            FROM test1.dbo.tableA AS A
            WHERE I.aForeignKey = A.PK
        )
    )
    BEGIN
        RAISERROR('Violation of fake constraint',16,1);
        ROLLBACK;
    END

END