When you are working on measurable goals for your child's IEP, and the assessment of those goals keeps being written as "teacher observation" as the only way to measure progress, you will never have concrete proof of student achievement. While teacher observation may be one way to measure progress, there are many more ways that can be discussed and added to your child's plan. Here are some responses if you start to hit this brick wall.
"I value the input of the teacher so much, and her ability to be in tune with my child's progress. In addition to that, I would like to discuss what are some other ways that we can assess this goal with objective measures, so that we can meaningfully compare the IEP goal with a measurable short term objective or benchmark."
"What kind of class work/homework can we monitor to ensure that this specific goal on the IEP will be addressed?"
"What assessments does the school already employ that we might use for this math goal?"