Now know we (νυν οιδαμεν). They had failed to understand the plain words of Jesus about going to the Father heretofore (John 16:5), but Jesus read their very thoughts (John 16:19) 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 John 16:27) and εκ του πατρος (verse John 16:28), απο, εκ, παρα all with the ablative of source or origin.