The speaker's musings on grief are mostly speculation—although she tries really hard to analyze the sadness of others, she gives no indication that she ever comes to conclusions about their suffering. While the poem begins with language that suggests she knows what's up, it actually ends with language that is less-than-certain, showing us that she's fumbling in the dark like the rest of us.
Lines 1-2: The diction the beginning of the poem is almost scientific or mathematical. Grief is being measured, and eyes are "probing" and "narrow."
Lines 7-8: The speaker admits to not knowing when her sadness began; all she knows it that it "feels so old." So this isn't all certain science—there's some doubt and confusion here, too.
Lines 3, 5, 9 and 17: The poem features anaphora of a key phrase, "I wonder" in order to highlight the importance of speculation, thought and imagination in the speaker's method.
Stanzas 9 and 10: While the poem's diction starts out as precise and mathematical, it actually ends by using language that is less-than-certain—"may not guess the kind / Correctly," "fascinated to presume," etc. The speaker suggests that she doesn't require complete knowledge at all—in fact, the less she knows, the better able she is to imagine others feeling the same way.