ἐν τῷ δὲ κεῖσαι: usu. explained, “"And on what sign of thine end dost thou rely?"” But “κεῖμαι ἔν τινι” (see on 247)=“"to be situated in a person's power"”: an analogous use of “κεῖμαι” here would give us, “"on what sign doth thy fate depend?"” In Tr. 82, however, we have “ἐν οὖν ῥοπῇ τοιᾷδε κειμένῳ”: and, if the text be sound, κεῖσαι has (I think) a like sense here: lit., “"at what sign of thy fate art thou in suspense?"” The phrase is thus virtually equivalent to “ἐν τίνι ῥοπῇ κεῖσαι”;—the τεκμήριον itself standing for the crisis which it marks. The phrase seems to me possible (for our poet), but slightly suspicious. We might conjecture καὶ τῷ πέπεισαι: cp. Eur. Hel. 1190 “ἐννύχοις πεπεισμένη
στένεις ὀνείροις
”. (To the obvious “κεῖται σοῦ, σόν”, or “σοί...τεκμήριον”, the objection is the phrase “ἐν τῷ κεῖται”.)