After A = 1; a = [1,2,A,4]; there is no method to detect, if a has been created using numbers, functions or other variables. When A is e.g. a character array, it is converted to double implicitly:
Here A is converted to the type DOUBLE and the ASCII value 90 is stored. Afterwards a is a double vector as all its elements have the same type.
I think, you have a different problem. Perhaps you want to test the class of A before inserting into the vector.