What is the rhythm in Sir Philip Sidney's poem "My True Love Hath My Heart"? How can it be explained? What are the clues?

The rhythm, or meter, of this poem is iambic pentameter. 
Iambic pentameter is the most common meter in British poetry (especially pre-twentieth
century), and it also the meter that most resembles natural speech in
English.

"Iambic pentameter" describes a line that is made
up of five ("pentameter") iambs.  An iamb is a grouping of two
syllables in which the first syllable is unstressed and the second syllable is
stressed.  A word in English that exemplifies iambic rhythm is "about" (among many
others), because we put the greater emphasis on the second syllable.  So, in this first
line by Sidney, we see the following rhythms (with the stressed syllable denoted by the
bold font):

My true love
hath my heart
and I have
his

In addition to sounding
the line out and counting the stresses, a good clue to figuring out the poem is written
in iambic pentameter is to look at the form.  This poem is a sonnet, as are all of
Sidney's other poems (Sidney's most famous work is his sonnet cycle Astrophil
and Stella).  Sonnets in English are always written in iambic
pentameter.