How to hire good designer?

I would not actually rely on a portfolio. I would rely on the level of participation, who is actually interested in this kind of career. At that point you are only gauging for that. They are way too young to have any meaningful body of work.

Beyond interest, I look for designers who have deep respect for engineering. I want designers who actually think about the person who will make their vision real. Without that person, your vision may not become reality. I also look for some interest in art. Artists are sloppy people, but their craft will be finished. They will still go and fix the last note they played wrongly. That last mile attitude is very critical in design, because if you do not have it, you will never build great products.