Large language models (LLMs) are increasingly used to produce open-ended, interpretive annotations, yet there is no validated, scalable measure of ***idea-level similarity*** to expert annotations. We (i) introduce the content evaluation of LLM annotations as a core, understudied task, (ii) propose IDEAlign for capturing expert similarity judgments via the odd-one-out tasks, and (iii) benchmark various similarity methods, such as text embeddings, topic models, and LLM-as-a-judge, against these human ratings. Applying this approach to two real-world educational datasets (interpreting math reasoning and feedback generation), we find that most metrics fail to capture the nuanced dimensions of similarity meaningful to experts. LLM-as-a-judge performs best (11–18% improvement over other methods) but still falls short of expert alignment, making it useful as a triage filter rather than a substitute for human review. Our work demonstrates the difficulty of evaluating open-ended LLM annotations at scale, and positions IDEAlign as a reusable protocol for benchmarking on this task, thereby informing responsible deployment of LLMs.