Google have now added spell checking to the new version of Google Sheets – a new and improved spell check compared to what was avilable within the old version.
Similarly to the implementation of this feature in Docs and Slides, spelling is checked via the web so that the dictionary is always up-to-date. Also like in Docs and Slides, people can add words to their own dictionary as I described in this post so that the spell checker won’t mark technical terms or other specialised markup items as misspellings. The personal dictionary is shared with Docs and Slides, so adding a word via any of these applications makes it available from the others.
You can access the spell checker by selecting Tools > Spelling.
Candidates for being incorrect words are featured in a window on the top right corner of the screen, along with suggestions for correct spellings, each of which can be accepted, ignored, or added to the custom dictionary.