Lee, if you can't educate your copywriters, there is not a lot you can do up front. Either disable automatic quotes and expect straight ones in your plain text, or enable them and you have to check every inch and foot mark (as well as minutes and seconds, by the way).
I think you're best off with enabling them, because you can use a GREP search to look for digit-then-curly-quote — the most common case where you want to change them. The opposite, checking which straight quotes to change to curlies, is a bit more difficult.
You can search for this, using GREP Find: for single quotes,
(?<=d)~]
and for double quotes,
(?<=d)~}
You can replace them with straight quotes from your text font (use the code ~' — that's a tilde, followed by a single straight quote, for feet, or ~” — tilde, straight double quotes for inch).
If you are inclined to typographically correctness, follow David's advice and insert the actual Symbol symbols. For these, you should replace with either x{2032} (single) or x{2033} (double), and don't forget to set the font to Symbol in the Replace With formatting field. (I lean towards ridiculous correctness and always create a character style containing just “Symbol”, especially for things like these.)
Whatever you change it to: do not hit the Replace All button straight away! Use “Change/Find Next” until you are fairly sure your document does not contain a phrase such as
“How old are you my child?” “I am 5″, answered Mary.
(It gets a bit difficult when that question is, “How tall are you my child?” Then you'd need some more context.)
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