I find this technique pretty easy...
Select the range
Format|cells|number tab|custom category
In the "type:" box, put this:
alt-0252;alt-0252;alt-0252;alt-0252
But hit and hold the alt key while you're typing the 0252 from the numeric
keypad.
It should look something like this when you're done.
ü;ü;ü;ü
(umlaut over the lower case u separated by semicolons)
And format that range of cells as Wingdings.
Now, no matter what you type (spacebar, x, anyoldtextatall), you'll see a check
mark.
Hit the delete key on the keyboard to clear the cell.
If you have to use that "checkmark" in later formulas:
=if(a1="","no checkmark","Yes checkmark")
And since this is just data, it'll sort (well, if you include that column in the
range).
And you can always filter for that funny character.