Is there a way to find out if the converted postfix expression is valid or not

Yes. We need to associate a rank for each symbol of the expression. The rank of an operator is -1 and the rank of an operand is +1. The total rank of an expression can be determined as follows:
- If an operand is placed in the post fix expression, increment the rank by 1.
- If an operator is placed in the post fix expression, decrement the rank by 1.
At any point of time, while converting an infix expression to a postfix expression, the rank of the expression can be greater than or equal to one. If the rank is anytime less than one, the expression is invalid. Once the entire expression is converted, the rank must be equal to 1. Else the expression is invalid.