How does one go about testing the flatness and trueness of v-ways (for example, those on the cross slide of a lathe) during the scraping process? I can't imagine being able to get at the surfaces with a straight edge like on bed ways.
If you can get to them to scrape them, you should be able to get to them to check them. Some disassembly may be (almost certainly will be) required. You may need a smaller straight edge, or you may be able to bring the part over to the surface plate for checking.