ἀφέμενον, ‘avoiding,’ lit. ‘letting go from oneself’, followed by the genitive in Attic Greek.
ἐπὶ τοῦ συνομολογουμένου λαμβάνειν αὐτῶν τὸν ἔλεγχον, ‘test them in the light of generally admitted principles.’ For ὁμολογεῖν see note on E. 44 ὁμολογία, and for this use of ἐπί cp. ἐπὶ τῶν χρησίμων, § 50 and note.
αὐτῶν: i.e. those whose advice Nicocles is thinking of taking, or perhaps quite generally of ‘mankind’.
μάλιστα μὲν ἐπὶ τῶν καιρῶν κτἑ, ‘if possible, you should observe them when they are giving advice to suit special occasions.’ μάλιστα often introduces the preferable of two alternatives. For καιρός see note on E. 31.
εἰ δὲ μή, καὶ καθ᾽ ὅλων τῶν πραγμάτων λέγοντας, ‘and failing that, when they are speaking on general topics.’
ἀποδοκίμαζε, ‘reject (after scrutiny)’; see note on δοκιμασίας, § 27.
ὁ μηδὲν ὢν αὐτὸς χρήσιμος, ‘if a man is of no use in himself.’ μηδέν is an adverbial accusative qualifying χρήσιμος.