Quarantine won't detect it usually. Only way to know is to test for it. It's kind of like aids in that respect, unless it's full blown aids, and not just hiv positive, they won't show any signs.
Be very careful swapping eggs here, or anywhere else for that matter. MG is everywhere and the best way to avoid it is to just pay the extra cash and buy from MG tested flocks, NPIP does not require it to be tested either, so don't think NPIP is safe either.