Put it right in the prototype. Use real content, not faked content. See, e.g., Death To Lorem Ipsum.
It is helpful to enumerate your goals here. What are you looking for when testing the written copy?
- Discoverable -- do people find it on the page?
- Engaging -- do people choose to read it, or do they gloss over it?
- Comprehensible -- do people understand what it says?
- Readable -- do people find it easy to read, or is it effort?
- Agreeable -- do people find it pleasant, neutral, or even offensive?
- Comprehensive -- does it cover all the topics you need?
- Effective -- does reading the copy change the way the user acts in the desired way?
These build on each other. It doesn't matter how readable it is if the user can't discover it.
Each of these have different goals, but they all have similar methods: present the content in situ, and ask the user about their task. If they do discover the text and read it, ask them about what they read to query understanding. If they don't, it's probably not discoverable enough; noting that, direct them to it and ask them what they think. And so forth--similar methods to the rest of usability testing.