I think that we can learn about assessment generally when we think about how education is done in art. Here, you can't just give students a multiple choice test, or even a specific problem to solve, because that's not what art is. Some things: you want to assess what they know, not what they've just learned. You need to leave room for them to take risks. You can't assess them on a single piece of work, but rather on a wider portfolio. And you're assessing what they put forward as worth assessing.