Usually it is no big deal to find an antecedent. Start looking for a word with the same number and gender as the pronoun. But every once in a while, the antecedent can be a little elusive.
In Phil 1, Paul explains how his imprisonment and all that has happened to him (τὰ κατ᾿ ἐμὲ) has served to advance the gospel throughout the Palace Guard, which in turn has emboldened the Philippian Christians (1:12–15). He then includes a short caveat, explaining that different people have different motives, but at the end of the day he concludes ἐν τούτῳ χαίρω (1:16–18a).
Paul then shifts tenses from the present χαίρω to the future χαρήσομαι to say that he will also rejoice because he knows (οἶδα γάρ) that “this” (τοῦτο) will turn out for his (μοι ἀποβήσεται) deliverance (or salvation, εἰς σωτηρίαν). What does τοῦτο refer to?