The first assumption is that if you have generated a mapped entity based on some table, that table should exist, otherwise something very bad is happening in your database
Also, if you fetch some entity/entitycollection and you obtain an exception saying that "you are querying an invalid object" then that could be a way.
You also can create some special stored procedure that inside query the sys/master tables to check for the existence of an object. You can call that stored procedure from llblgen.