Jamieson, Fausset and Brown on Ecclesiastes 6:12

12. For who knoweth, &c.--The ungodly know not what is really "good" during life, nor "what shall be after them," that is, what will be the event of their undertakings (Ec 3:22; 8:7). The godly might be tempted to "contend with God" (Ec 6:10) as to His dispensations; but they cannot fully know the wise purposes served by them now and hereafter. Their sufferings from the oppressors are more really good for them than cloudless prosperity; sinners are being allowed to fill up their measure of guilt. Retribution in part vindicates God's ways even now. The judgment shall make all clear. In Ec 7:1-29, he states what is good, in answer to this verse.