How can I prove to employers that are looking for mathematicians, that I can do maths?

I am studying for a computer engineering degree, however, I've been going to the library every night for the past year, studying mathematics. I went through four complete undergraduate textbooks in calculus, linear algebra, analysis, and statistics, plus some more advanced time series stuff. I went through the past papers for my university's 3rd year undergraduate maths course and I was able to answer the questions quite comfortably.

But the thing is, I don't have a degree in maths, nor am I studying for one.