I do not know a universal rule but one. If the alphabet’s element number is 1 then all context free grammars are automatically regular.
In other cases, however, I would try experimenting. I’d look at the grammar, I’d figure out the language, I’d try to substitute the grammar by another… &so. Somehow I’d guess if the stack had a limit on depth. <a href='https://wordmaker.info/how-many/grammar.html'>grammar</a>