There is no need to "guess" the rhyme in poetry.
Look at the last word in each line of the poem you are working on.
Mark the first line with an a and then find the next line where the last word rhymes with this word. Mark that one with an a as well. It may be the second line or it may be the third or fourth.
Then look at the second line. If that rhymed with your first then of course go to the third line and so on. Mark that new sound at the end of the line with a b and find the next line where the last word rhymes with it and mark that one with a b.
You will begin to see a pattern emerge. The common patterns are (aabb) (ccdd) or (abba) (cddc) and sometimes followed by (ee). Some poems have the same rhyme scheme throughout.
Here's an example:
In the small African village of Matabwo a
Where the willows sway to and fro a
The people lived happily together b
And never worried about the weather b
It is a way to give the poem a rhythm and represents the poet's style.