I suppose the easiest thing to check is if the text evaluated to a result.
Excell could evaluate the cell and if it didn't return an error, then it
should add an equal sign. This would give some false positives, for instance
"3", but "=3" would be the same as "3" in most casses of which I can think.
Another option wuold be to check the cell for specific patterns, for
instance, +, -, /, Excel function names. I realize there are instances in
which these are not meant to be a part of a formula, but most cases they are
meant to be a part of a formula.
In a related subject, It would be great if the autocorrect for spelling
worked in Excel as well. Also, why not red squigly underline spellinging
errors in the forumla bar (or in the sheet).