There is no support for spell checking in WireframeSketcher at this point. To add something like this we need a generalized support for spell checking in Eclipse platform, which so far hasn't been implemented. You can vote for this feature here: https://bugs.eclipse.org/bugs/show_bug.cgi?id=185695
Other solution that I could suggest is using a command line tool like aspell, that you can find on Linux, but on other platforms too. Since WireframeSketcher screen files are saved in XML format, aspell can check them if used with correct settings that interprets XML correctly.