Unfortunately, seems the only way to figure this out is by testing. But on the flip, nobody can really trust that it won't accidentally or purposefully escape from labs.
They can create a few mirrored proteins and sugars and test them. Without RNA (and a lot of additional machinery) the proteins and sugars will not start to spontaneusly reproduce to conquer the word.
They can "reproduce" only in very specific environmens where there are similar non-malformed proteins nearby. It would be very hard to have a similar problem with a mirrored molecule that picks one type of normal molecules and mirror them.
We really can't. Militaries in particular aren't into bioweapons. Too hard to aim. They're pretty much strictly the domain of terrorists, and terrorists don't have the resources to execute attacks on the bleeding edge of biotech.
Remember, no one has built a regular cell from scratch, much less come close to building a mirror cell.
For defensive purposes, FWIW. I suppose they might create some mirror molecules to test defenses, but I believe gus_massa's point stands that you don't need to generate full, functioning cells for that purpose.