How do we identify the molecular formula from the empirical formula of a completely unknown substance?
If the empirical formula is known, then any method that will determine the molecular weight will do. Of merely historical interest is the use of freezing-point depression, nowadays, one would use mass spectrometry (electron-spray ionisation will leave most molecules intact).

This assumes that it is a reasonably small molecule, but only for those will the empirical formula be known before the molecular one.