I would suggest to prepend all versions (whether it's compressed or not) by one character. Let's say if you compress you prepend the result with 'z' and if you don't, you prepend with '_' for example. Then, on the decompress side, you check the first char of your string and decide if you should decompress or not depending on this char.
That said, if you plan on storing those strings locally (in the browser), no matter what size is you string you'll most likely get a smaller version with LZString.
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