Someone else may know more about this, so please comment if you do.
For me the easiest thing is to use an internet browser that has a spell-check feature, like Mozilla Firefox. This is from their website:
"Firefox automatically checks the spelling of words that you enter in
text boxes containing more than one line. As soon as you finish typing a
word, it is checked against the words in the installed dictionary. If
the word is not found in the dictionary, it will be underlined in red."
It works for me like this: after typing a reply, I hit the "Preview" button to see how the post will look. Misspelled words appear with a squiggly red line underneath. Sometimes it's helpful to right-click while you're in the text box and make sure the "check spelling" option is selected (it will show a check mark next to it -- if not, click on it and the check should appear).
This isn't a great option, but it might help until we get something better.