This is just my opinion, but the word quality is a "generic" term. But if you had to judge, a number of things that may be good to look at, is:

1) Are they commenting their code?

2) Formatting structure of code

3) Are there redundancies in logic, that over complicate a simple idea.

4) Are they using "common" best practices while developing the code?

5) How long is it taking them to create/implement base elements like functions, classes, variables, etc.

These are just some ideas to look at. A lot of programmers are really great at what they do, but lack organization of what they build. Some programmers are great at organization, but over complicate simple ideas. It's variable based on what your "expecting" with the end product.

Do you want, just get it done, and it works code?

Do you want clean, efficient, and best practices code?