Jamieson, Fausset and Brown on Ephesians 5:15

15. that--rather as Greek, "See how ye walk," &c. The double idea is compressed into one sentence: "See (take heed) how ye walk," and "See that ye walk circumspectly." The manner, as well as the act itself, is included. See how ye are walking, with a view to your being circumspect (literally, accurate, exact) in your walk. Compare Col 4:5, "Walk in wisdom (answering to 'as wise' here) toward them that are without" (answering to "circumspectly," that is, correctly, in relation to the unbelievers around, not giving occasion of stumbling to any, but edifying all by a consistent walk).

not as fools--Greek, "not as unwise, but as wise."