What is one characteristic of an iambic pentameter used in sonnet 29?
Your question asks about characteristics of iambic pentameter. The characteristics of iambic pentameter are described in its very definition. It is a metrical pattern in which a line of poetry contains five metric feet (because the prefix penta means "five", therefore penta-meter contains five units), and in which each of these "feet" is made of two syllables, with the stress upon the second syllable in each foot. This type of foot is called an iamb.
Perhaps you meant to ask for examples of the iambic pentameter structure as they are found in Sonnet 29. The good news is that most of the time sonnets in English employ iambic pentameter, so it is easy to pull out several examples of lines that are built of five iambs.
To determine whether a line is totally iambic, it is helpful to say it aloud to yourself as you look at it, and mark its scansion if you have a printed or otherwise markable copy of it. Scansion is the process of dividing lines of poetry into their metric feet and then marking the stress pattern of stressed or unstressed syllables in them. If we look at the text of Shakespeare's Sonnet 29, it is easy to spot them. Let's look at the patterns in the first four lines of it, which are:
When, in disgrace with fortune and men's eyes,
I all alone beweep my outcast state,
And trouble deaf heaven with my bootless cries,
And look upon myself, and curse my fate,
If we divide this text into its feet and then identify the stressed syllables, it should show a pattern like this (where bolded syllables are the stressed, and non-bolded syllables are the unstressed):
When, in / disgrace / with for / tune and / men's eyes,
I all / alone / beweep / my out / cast state,
And trou / ble deaf heav / en with / my boot / less cries,
And look / upon / myself, / and curse / my fate,
Note how each line is divided into five sections, and that in most of these there are only two syllables. Lines 1, 2, and 4 all fit the iambic requirement of two syllables in each foot and stress being on the second (last) syllable in the foot. But in line 3 there is a slight deviation. The second foot has three syllables in it instead of two, so it is not an iamb. That foot's pattern is called anapest rather than iamb. If you were to continue marking the iambic pattern of stresses and unstressed syllables for the rest of the poem's lines, you would find that all together lines 1, 2, 4, 7, 8, 12, 13, and 14 conform strictly to iambic stress. But lines 3, 5, 6, 9, 10, and 11 contain exceptions such as an extra syllable crammed into the line (so it has eleven instead of ten), or stress falling on the first syllable rather than the second. For example, when line 5 begins with "Wishing", one feels compelled to put the emphasis on the "Wish" part of it rather than pronouncing it as wishING. Likewise in line 10 the first word "Haply" sounds more natural with emphasis on the first rather than the second syllable. We would say HAP-ly rather than hap-LY.