The field in table 2 has to be indexed in order for it to check. If its an unstored field or calc then it will not work.
I am trying to validate data entry in a field in table "1" by checking that the value exists in a field in table "2".
1) create a value list with values coming from field Table2::YourField ( that field must be indexable )
2) validate Table1:: Otherfield as member of that value list