Exegesis:

kata ti gnōsomai touto lit. ‘in accordance with what,’ i.e. ‘by what sign shall I know this?.’

ginōskō ‘to come to know,’ ‘to know.’ Here it means ‘to be sure of,’ cf. New English Bible, Phillips, Bible de Jérusalem.

egō gar eimi presbutēs ‘for I am an old man.’ The connexion with the preceding clause is as follows: Zechariah requested a sign because he could not believe what the angel had told him, and he could not believe it because he and his wife were old.

presbutēs † ‘old man,’ substantive.

probebēkuia en tais hēmerais autēs ‘advanced in her days,’ cf. on v. 7.

Translation:

Said, often better, ‘asked,’ because the verb introduces a question. A similar shift must often be considered before direct questions, e.g. in 1.34; 7.19 (“sent saying”, i.e. ‘sent to ask him’); 7.40 (“answering, said”, i.e. ‘in his turn asked’); and, though less common, also before rhetorical questions, or exclamations having the form of a question, such as 2.48; 4.22, 36, etc.

How shall I know this?, or, ‘how (or, by what means, or, by what sign) shall I know this is true,’ ‘how (etc.) can I be sure of this (or, that this will happen),’ ‘how shall the certainty of this be’ (Tboli), ‘relying on what can I know this thing’ (Chinese).

Advanced in years, see on v. 7. If there ‘old/aged’ has been used, as being the closest natural equivalent, this rendering will sound repetitive in the present verse. Tagalog solves the problem by saying, ‘I am very old already, and like that also is my wife.’

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.