I think the only way to fully know would be to test him on comprehension and word meaning, and have him read out loud and track the decoding errors. It's still hard though because how do you standardize your test? I don't think there's an exact answer. The school will do their thing, and unless you get testing materials from the teacher (reading samples, questions, etc.) it can be hard to duplicate.
If it were me, I'd google to see what grade level the book is at, and then use that as an approximation.
I know what you mean though... you hear lots of "my child reads at this level" ...but what does that mean?
