The exegetical guide says that the verb mood of "know" in 1 John 4:2 is imperative, but none of the Bible translations translate this verse as a command.
Intriguing. The KJV does seem to translate it in the imperative:
"Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God."
Bultmann (today's reading, indeed, this verse!):
"Γινώσκετε (“you know”) can be understood as an imperative, but probably better as an indicative, just as in 3:20* the readers are reminded of the knowledge which they possess.5 It is the knowledge that πᾶν πνεῦμα ὃ ὁμολογεῖ Ἰησοῦν Χριστὸν ἐν σαρκὶ ἐληλυθότα ἐκ τοῦ θεοῦ ἐστιν (“every spirit which confesses Jesus Christ as having come in the flesh is of God”)"
The +/- sign in the guide is the indicator that there are multiple parsings depending upon who's expertise you consider. Any decision as to which is "correct" belongs after a thorough review of the literature to understand why those with expertise in the field disagree, a brief review of the textual variants, and some time to mull over, weigh, and come to a solid argument in support of the position you chose to take. For the rest of us, I recommend simply keeping in mind that it is a disputed issue that has little practical effect on how I worship and behave. (Yes, I'm in one of my "moods" this morning as a result of broadening my reading with some really horrendous theology).
