If I say I “know” I am saved, how do I know? How would I assure someone else that I am assured of my salvation?

It is true that some who are true Christians are not assured of their true status:

“One may say that he is persuaded that nothing present, or to come, can separate him from the love of God (Rom 8:18). Another comes but this far, ‘Lord, I believe; help thou mine unbelief’” (Mark 9:24) Puritan Daily Devotional Chronicles (ref#225, Oct 15th).

And it is also true that some who are not Christians are assured that they are:

“Many have lived in an outward profession; they have put on Christ with external garb, have talked well of Him, have been baptized in His name, have given liberally for His cause…” Octavius Winslow (ref#135, Aug 10th).