Question: Check if there is a non-numerical entry in my Matrix

Hello Everyone,

I am currently in the process of building a user interface where a user can input a ply layup for a composite part which needs to be calculated. I want to forbid the use of non-numerical value in the matrix which represents my ply layup, before it gets sent to be calculated, but I am wondering how to check if my matrix has non-numerical values in it. 

At best, it would not only check if the matrix has non-numerical entries, but also give me the indices where exactly that value is present. I have yet been unsuccessful trying to find a fitting command. Do you perhaps know something that might work.
For clairification, allowable entries are in a range of -90 to 90, so it should not look for the "-" in the matrix. 

Best Regards,
Lennart