A.T. Robertson Commentary John 16:30

"Now know we that thou knowest all things, and needest not that any man should ask thee: by this we believe that thou camest forth from God." — John 16:30 (ASV)

Now know we (νυν οιδαμεν). They had failed to understand the plain words of Jesus about going to the Father heretofore (16:5), but Jesus read their very thoughts (16:19f.) and this fact seemed to open their minds to grasp his idea.

Should ask (ερωτα). Present active subjunctive with ινα in original sense of asking a question.

By this (εν τουτω). In Christ's supernatural insight into their very hearts.

From God (απο θεου). Compare παρα του πατρος (verse 27) and εκ του πατρος (verse 28), απο, εκ, παρα all with the ablative of source or origin.