Justin was evidently born in Flavia Neapolis to pagan Greek parents. He wrote in Greek and no doubt understood the language very well. As Quasten observes:
"The most important of the Greek apologists of early Christian literature is Justin the Martyr" (Johannes Quasten, Patrology: The Beginnings of Patristic Literature, 1:196).
As far as analyzing words goes, Justin gives plenty of analysis in his writings. This can easily be seen by consulting the Greek text of Justin's Dialogue with Trypho, if you can locate one. An excellent study in this regard is Oskar Skarsaune's The Proof from Prophecy - A Study in Justin Martyr's Proof-Text Tradition: Text-Type, Provenance, Theological Profile (Leiden: Brill, 1987).
Skarsaune compares a number of texts from Justin's writings with the LXX. Admittedly, at times it appears that the Martyr is quoting the LXX from memory; at other times, it seems that he is relying on a testimonial source of some kind. But there are numerous places in Justin's works where there is no doubt that he is quoting from the LXX itself.
"Turning to Dial. 87.2, we find Is 11:1-3 quoted in a pretty good LXX text. The only significant variants are in vs. 1, where Justin's text is somewhat smoother in its parallelism than the LXX" (Skarsaune, 52).
Καὶ ἐξελεύσεται ῥάβδος ἐκ τῆς ῥίζης Ιεσσαι, καὶ ἄνθος ἐκ τῆς ῥίζης ἀναβήσεται. (Isaiah 11:1, LXX)
Καὶ ἐξελεύσεται ῥάβδος ἐκ τῆς ῥίζης Ιεσσαι καὶ ἄνθος ἀναβήσεται ἐκ τῆς ῥίζης Ιεσσαι (Dial 87.2)
I also recommend Moises Silva and Karen H. Jobes' Invitation to the Septuagint(Grand Rapids: Baker Academic, 2000).