Is "Archaic Torso of Apollo," by Rainier Maria Rilke, a sonnet or Petrarchan sonnet? How would I identify enjambment or iambic pentameter in the poem?

I would classify Rainier Maria Rilke's poem as a Petrarchan sonnet. Some Petrarchan sonnets consist of two quatrains (stanzas with four lines) and two tercets (stanzas with three lines), which is what we have here. Of course, all sonnets consist of fourteen lines, and Rilke's poem is no exception. Classic Petrarchan sonnets use the octave (eight lines) followed by a sestet (six lines). Whichever of the two forms a poet uses, it is clear the typical Petrarchan sonnet will consist of a combination eight line-six line pattern.
The poem is also written in iambic pentameter (more on that in a bit), another feature of a Petrarchan sonnet. Typically, quatrains follow the abba abba rhyme scheme, while sestets follow a varying cdcdcd, cdecde, or even cdccdc rhyme scheme. Generally, the rhyme scheme for the sestets are extremely flexible. If we look at the English translation of Rilke's poem, it can look like the sonnet is written in free verse. However, if we peruse Rilke's poem in German, it's quite clear that we have a Petrarchan sonnet on our hands.
Just a note that, while the typical rhyme scheme for the two quatrains in a Petrarchan sonnet generally follow the abba abba pattern, the German version of Rilke's poem below follows the abba cddc rhyme pattern (a little unusual but still a characteristic of the Petrarchan sonnet). Contrast this with the typical Shakespearean sonnet rhyme scheme which is abab cdcd efef gg.

Wir kannten nicht sein unerhörtes Haupt, (a)darin die Augenäpfel reiften. Aber (b)sein Torso glüht noch wie ein Kandelaber, (b) in dem sein Schauen, nur zurückgeschraubt, (a) sich hält und glänzt. Sonst könnte nicht der Bug (c)der Brust dich blenden, und im leisen Drehen (d)der Lenden könnte nicht ein Lächeln gehen (d)zu jener Mitte, die die Zeugung trug. (c)Sonst stünde dieser Stein entstellt und kurz (e)unter der Schultern durchsichtigem Sturz (e)und flimmerte nicht so wie Raubtierfelle (f)und bräche nicht aus allen seinen Rändern (g)aus wie ein Stern: denn da ist keine Stelle, (f)die dich nicht sieht. Du mußt dein Leben ändern. (g)

In the English translation, however, it's difficult to keep to the typical Petrarchan rhyme scheme. Therefore, it is the German original we must look at in order to determine whether this poem is a Petrarchan sonnet.
The English translation looks like this in the two quatrains:

We cannot know his legendary headwith eyes like ripening fruit. And yet his torsois still suffused with brilliance from inside,like a lamp, in which his gaze, now turned to low,gleams in all its power. Otherwisethe curved breast couldnot dazzle you so, nor could a smile run through the placid hips and thighsto that dark center where procreation flared.

There is definitely evidence of iambic pentameter in the poem. Iambic pentameter consists of ten syllables in a line, divided into five iambs of unstressed and stressed syllables. I will show this from the first line of the first quatrain:

We can/not know/ his leg/enda/ry headWir kann/ten nicht/ sein un/erhört/es Haupt

Now, in the English translation, some lines consist of more than ten syllables. As a result, it's always best to look at the poem in German, if at all possible. As for enjambment, we can see evidence of this even in the English translation. Enjambment consists of lines that don't end in punctuation. In a sense, one line runs into the next. The second stanza of Rilke's poem is a great example of this.

...gleams in all its power. Otherwisethe curved breast could not dazzle you so, nor coulda smile run through the placid hips and thighsto that dark center where procreation flared.

Last, but not least, Petrarchan sonnets also present conflicts or questions in the first eight lines (or two quatrains); in the last six lines (or two tercets), the solution presents itself. Rilke's poem is no exception. In the first two quatrains, he talks about a statue that has no head and therefore, no eyes. Its torso is "suffused with brilliance from inside," and this allows the statue to train its gaze on us. Furthermore, the juncture where the hips and thighs meet is creased into a "smile" of sorts. The "dark center where procreation flared" is very much sensually present still.
The two tercets present a solution. In order to see the statue in a new light, we must change our perspective

. . . for here there is no place that does not see you. You must change your life.