Siegfried Sassoon's poem includes several lines that conform to the iambic tetrameter pattern. Iambic tetrameter is a metrical pattern in which the line of poetry is divided into four feet (iambs) with stress placed upon the second syllable in each of those feet. If you were to divide the lines of poetry into chunks of two syllables each, you would then be able to examine whether the second syllable in each bears more stress.
The first four lines can be diagrammed as follows, split into iambs. I have used bold font to indicate which syllables are stressed, and left the unstressed syllables not in bold font:
I knew / a sim / ple sol / dier boy
Who grinned / at life / in emp / ty joy,
Slept sound / ly through / the lone / some dark,
And whist / led ear / ly with / the lark.
As you can see, all four of the lines in this first stanza conform to the iambic tetrameter pattern. It is important to look at the rest of the lines in the poem too, though, because not all of them employ iambic tetrameter. In case of one or more lines deviating from the pattern, you would need to be able to identify which of them are not good examples of iambic tetrameter. Consider the second stanza:
In win / ter trench / es, cowed / and glum,
With crumps / and lice / and lack / of rum,
He put / a bull / et through / his brain.
No one / spoke of him / again.
The last line of this stanza does not divide neatly into groups of two syllables because there are seven syllables rather than an even number. Furthermore, if the line is spoken aloud it sounds awkward to try and put the stress onto the second, fourth, and sixth syllable like you would have to if they were part of iambs. Saying "No one / spoke of / him a / gain" just does not sound right, and it leaves the syllable "gain" hanging all by itself at the end. So in stanza two, the lines of iambic tetrameter are lines five, six, and seven, but not line eight. Line eight appears to contain a trochee, a dactyl, and an iamb.
After that group of lines, we then move on to stanza three. In this final stanza all four lines are in iambic tetrameter:
You smug- / faced crowds / with kind / ling eye
Who cheer / when sol / dier lads / march by,
Sneak home / and pray / you'll nev / er know
The hell / where youth / and laugh / ter go.