This verse is closely connected with verse 27, about which see the comments.

The word translated examine is related to the adjective in verse 19 translated “genuine” and has a similar meaning: “check whether all is right.” To “test oneself,” as in Revised English Bible, Moffatt, and Die Bibel im heutigen Deutsch, is perhaps too narrow, as it suggests just one particular test. Some languages may have to say something like “look carefully into the heart,” in order to avoid the idea of a physical examination.

And so: the so is emphatic, implying “and only when he has examined himself.” This meaning is made explicit by Die Bibel im heutigen Deutsch, “… before he eats the bread”; Parola Del Signore: La Bibbia in Lingua Corrente says “Each one therefore should first examine himself, and then eat…”; and Revised English Bible, “everyone must test himself before eating from the bread….”

Quoted with permission from Ellingworth, Paul and Hatton, Howard A. A Handbook on Paul’s First Letter to the Corinthians, 2nd edition. (UBS Handbook Series). New York: UBS, 1985/1994. For this and other handbooks for translators see here .